diff --git a/app/src/androidTest/java/org/libremediaconverter/bench/RealMediaBenchmark.kt b/app/src/androidTest/java/org/libremediaconverter/bench/RealMediaBenchmark.kt
index f103d38..668fc6b 100644
--- a/app/src/androidTest/java/org/libremediaconverter/bench/RealMediaBenchmark.kt
+++ b/app/src/androidTest/java/org/libremediaconverter/bench/RealMediaBenchmark.kt
@@ -100,7 +100,7 @@ class RealMediaBenchmark {
         val hwOut = File(context.cacheDir, "bench_hw.mp4").apply { delete() }
         val engine = Media3Engine(context)
         val hwMs = try {
-            runCatching { timed { engine.transcode(uri, hwOut, MimeTypes.VIDEO_H265) } }
+            runCatching { timed { engine.transcode(uri, hwOut, OutputFormat.MP4_H265) } }
                 .onFailure { Log.w(TAG, "BENCH hardware: UNSUPPORTED (${it.message})") }
                 .getOrNull()
         } finally {
@@ -162,7 +162,7 @@ class RealMediaBenchmark {
         val out = File(context.cacheDir, "bench_av1_out.mp4").apply { delete() }
         val engine = Media3Engine(context)
         val ms = try {
-            timed { engine.transcode(Uri.fromFile(input), out, MimeTypes.VIDEO_H265) }
+            timed { engine.transcode(Uri.fromFile(input), out, OutputFormat.MP4_H265) }
         } finally {
             engine.close()
         }
diff --git a/app/src/androidTest/java/org/libremediaconverter/convert/Media3EngineTest.kt b/app/src/androidTest/java/org/libremediaconverter/convert/Media3EngineTest.kt
index dc28d79..82577c7 100644
--- a/app/src/androidTest/java/org/libremediaconverter/convert/Media3EngineTest.kt
+++ b/app/src/androidTest/java/org/libremediaconverter/convert/Media3EngineTest.kt
@@ -11,10 +11,12 @@ import kotlinx.coroutines.runBlocking
 import kotlinx.coroutines.withTimeout
 import org.junit.After
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
 import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
+import org.libremediaconverter.model.OutputFormat
 import java.io.File
 import java.util.concurrent.Executors
 import java.util.concurrent.TimeUnit
@@ -60,7 +62,7 @@ class Media3EngineTest {
         engine.transcode(
             input = Uri.fromFile(input),
             output = output,
-            videoMimeType = MimeTypes.VIDEO_H265,
+            format = OutputFormat.MP4_H265,
         ) { percent -> seen += percent }
 
         assertTrue("export produced no file", output.exists())
@@ -77,6 +79,35 @@ class Media3EngineTest {
         seen.forEach { assertTrue("progress out of range: $it", it in 0..100) }
     }
 
+    /**
+     * Regression guard for the audio-extraction bug.
+     *
+     * [OutputFormat.M4A_AAC] declares `VideoCodec.NONE`, and the router sends it to Media3. But
+     * the engine used to build a bare `EditedMediaItem` and take a video MIME type that defaulted
+     * to HEVC, so "extract the audio" transcoded the *video* to H.265 and wrote it to a file named
+     * `.m4a`. Nothing failed; the output was simply not what was asked for.
+     *
+     * This is the test the suite was missing — [Media3EngineTest] had no audio-only case at all,
+     * which is why the defect survived.
+     */
+    @Test
+    fun audioOnlyExportDropsTheVideoTrack(): Unit = runBlocking {
+        val audio = File(context.cacheDir, "out_audio.m4a")
+        audio.delete()
+        try {
+            engine.transcode(Uri.fromFile(input), audio, OutputFormat.M4A_AAC)
+
+            assertTrue("export produced no file", audio.exists() && audio.length() > 0)
+            val tracks = trackMimeTypesOf(audio)
+            assertEquals("expected exactly one track, got $tracks", 1, tracks.size)
+            assertEquals(MimeTypes.AUDIO_AAC, tracks.single())
+            assertNull("an audio-only export must carry no video track", videoMimeTypeOf(audio))
+            assertTrue("output has no duration", durationMsOf(audio) > 0)
+        } finally {
+            audio.delete()
+        }
+    }
+
     /**
      * Regression guard for the Transformer threading trap.
      *
@@ -124,7 +155,7 @@ class Media3EngineTest {
         val failure = runCatching {
             runBlocking {
                 withTimeout(30_000) {
-                    engine.transcode(Uri.fromFile(input), impossible, MimeTypes.VIDEO_H265) {}
+                    engine.transcode(Uri.fromFile(input), impossible, OutputFormat.MP4_H265) {}
                 }
             }
         }.exceptionOrNull()
@@ -148,6 +179,17 @@ class Media3EngineTest {
         }
     }
 
+    private fun trackMimeTypesOf(file: File): List<String> {
+        val extractor = MediaExtractor()
+        return try {
+            extractor.setDataSource(file.absolutePath)
+            (0 until extractor.trackCount)
+                .map { extractor.getTrackFormat(it).getString(MediaFormat.KEY_MIME).orEmpty() }
+        } finally {
+            extractor.release()
+        }
+    }
+
     private fun videoMimeTypeOf(file: File): String? {
         val extractor = MediaExtractor()
         try {
diff --git a/app/src/androidTest/java/org/libremediaconverter/fallback/FakeFailures.kt b/app/src/androidTest/java/org/libremediaconverter/fallback/FakeFailures.kt
index 84b93e8..f2839d4 100644
--- a/app/src/androidTest/java/org/libremediaconverter/fallback/FakeFailures.kt
+++ b/app/src/androidTest/java/org/libremediaconverter/fallback/FakeFailures.kt
@@ -7,6 +7,7 @@ import org.libremediaconverter.convert.HardwareTranscoder
 import org.libremediaconverter.convert.OutputPublisher
 import org.libremediaconverter.convert.SoftwareTranscoder
 import org.libremediaconverter.model.ConversionRequest
+import org.libremediaconverter.model.OutputFormat
 import java.io.File
 
 /**
@@ -24,7 +25,7 @@ object FakeFailures {
         override suspend fun transcode(
             input: Uri,
             output: File,
-            videoMimeType: String,
+            format: OutputFormat,
             onProgress: (Int) -> Unit,
         ) {
             called = true
diff --git a/app/src/main/java/org/libremediaconverter/convert/Media3Engine.kt b/app/src/main/java/org/libremediaconverter/convert/Media3Engine.kt
index bb99468..0c78040 100644
--- a/app/src/main/java/org/libremediaconverter/convert/Media3Engine.kt
+++ b/app/src/main/java/org/libremediaconverter/convert/Media3Engine.kt
@@ -15,6 +15,9 @@ import androidx.media3.transformer.ProgressHolder
 import androidx.media3.transformer.Transformer
 import kotlinx.coroutines.CancellableContinuation
 import kotlinx.coroutines.suspendCancellableCoroutine
+import org.libremediaconverter.model.AudioCodec
+import org.libremediaconverter.model.OutputFormat
+import org.libremediaconverter.model.VideoCodec
 import java.io.File
 import kotlin.coroutines.resume
 import kotlin.coroutines.resumeWithException
@@ -55,12 +58,19 @@ class Media3Engine(private val context: Context) : HardwareTranscoder {
     override suspend fun transcode(
         input: Uri,
         output: File,
-        videoMimeType: String,
+        format: OutputFormat,
         onProgress: (Int) -> Unit,
     ): Unit = suspendCancellableCoroutine { cont ->
         handler.post {
-            val transformer = buildTransformer(videoMimeType, cont)
-            val item = EditedMediaItem.Builder(MediaItem.fromUri(input)).build()
+            val transformer = runCatching { buildTransformer(format, cont) }
+                .getOrElse { cont.resumeWithException(it); return@post }
+            // Dropping the tracks the target format does not have is what stops an audio-only
+            // export from carrying a re-encoded video track. Without setRemoveVideo, asking for
+            // M4A produced an HEVC stream in a file named .m4a.
+            val item = EditedMediaItem.Builder(MediaItem.fromUri(input))
+                .setRemoveVideo(format.videoCodec == VideoCodec.NONE)
+                .setRemoveAudio(format.audioCodec == AudioCodec.NONE)
+                .build()
 
             cont.invokeOnCancellation {
                 // cancel() has the same single-thread requirement as start().
@@ -74,26 +84,69 @@ class Media3Engine(private val context: Context) : HardwareTranscoder {
         }
     }
 
+    /**
+     * @throws IllegalArgumentException if [format] names a container Media3 cannot mux. That is a
+     *   routing bug rather than a runtime condition — [org.libremediaconverter.model.ConversionRouter]
+     *   is supposed to have sent such a job to FFmpeg — so it fails loudly instead of quietly
+     *   writing MP4, which is what the old code did.
+     */
     private fun buildTransformer(
-        videoMimeType: String,
+        format: OutputFormat,
         cont: CancellableContinuation<Unit>,
-    ): Transformer = Transformer.Builder(context)
-        .setLooper(thread.looper)
-        .setVideoMimeType(videoMimeType)
-        .addListener(object : Transformer.Listener {
-            override fun onCompleted(composition: Composition, result: ExportResult) {
-                if (cont.isActive) cont.resume(Unit)
-            }
+    ): Transformer {
+        val muxerFactory = requireNotNull(Media3Muxers.factoryFor(format.container)) {
+            "Media3 cannot mux ${format.container}; this job should have routed to FFmpeg."
+        }
 
-            override fun onError(
-                composition: Composition,
-                result: ExportResult,
-                exception: ExportException,
-            ) {
-                if (cont.isActive) cont.resumeWithException(exception)
-            }
-        })
-        .build()
+        val builder = Transformer.Builder(context)
+            .setLooper(thread.looper)
+            .setMuxerFactory(muxerFactory)
+
+        // Only name a MIME type for a track the output actually keeps. Naming one for a removed
+        // track makes Transformer build an encoder for samples that will never arrive.
+        videoMimeTypeFor(format.videoCodec)?.let(builder::setVideoMimeType)
+        audioMimeTypeFor(format.audioCodec)?.let(builder::setAudioMimeType)
+
+        return builder
+            .addListener(object : Transformer.Listener {
+                override fun onCompleted(composition: Composition, result: ExportResult) {
+                    if (cont.isActive) cont.resume(Unit)
+                }
+
+                override fun onError(
+                    composition: Composition,
+                    result: ExportResult,
+                    exception: ExportException,
+                ) {
+                    if (cont.isActive) cont.resumeWithException(exception)
+                }
+            })
+            .build()
+    }
+
+    /**
+     * Media3 encodes only H.264 and H.265 of the codecs this app offers.
+     *
+     * VP8/VP9/AV1 targets never reach here — the router sends them to FFmpeg because
+     * `Transformer.setVideoMimeType` rejects them — so anything unexpected returns null and lets
+     * Transformer pick, rather than silently substituting H.265 the way the old mapping did.
+     */
+    private fun videoMimeTypeFor(codec: VideoCodec): String? = when (codec) {
+        VideoCodec.H264 -> MimeTypes.VIDEO_H264
+        VideoCodec.H265 -> MimeTypes.VIDEO_H265
+        VideoCodec.NONE -> null
+        VideoCodec.VP8, VideoCodec.VP9, VideoCodec.AV1 -> null
+    }
+
+    private fun audioMimeTypeFor(codec: AudioCodec): String? = when (codec) {
+        AudioCodec.AAC -> MimeTypes.AUDIO_AAC
+        AudioCodec.OPUS -> MimeTypes.AUDIO_OPUS
+        AudioCodec.VORBIS -> MimeTypes.AUDIO_VORBIS
+        AudioCodec.PCM -> MimeTypes.AUDIO_RAW
+        AudioCodec.NONE -> null
+        // MP3 and FLAC have no Android encoder; the router routes them to FFmpeg.
+        AudioCodec.MP3, AudioCodec.FLAC -> null
+    }
 
     /**
      * Polls export progress on the Transformer's own thread.
diff --git a/app/src/main/java/org/libremediaconverter/convert/Media3Muxers.kt b/app/src/main/java/org/libremediaconverter/convert/Media3Muxers.kt
new file mode 100644
index 0000000..d9a65ac
--- /dev/null
+++ b/app/src/main/java/org/libremediaconverter/convert/Media3Muxers.kt
@@ -0,0 +1,60 @@
+package org.libremediaconverter.convert
+
+import androidx.media3.common.util.UnstableApi
+import androidx.media3.muxer.Muxer
+import androidx.media3.transformer.DefaultMuxer
+import org.libremediaconverter.model.Container
+
+/**
+ * Which containers Media3 can actually write, and why it is only one.
+ *
+ * ## The four other muxers cannot be driven by Transformer
+ *
+ * `media3-muxer` 1.11.0 ships `WebmMuxer`, `OggMuxer`, `WavMuxer` and `AacMuxer` alongside the MP4
+ * ones, which reads like four more containers on the hardware path. It is not: **all four throw
+ * `UnsupportedOperationException` from `addMetadataEntry`**, and
+ * `MuxerWrapper.addTrackFormat` calls it for every metadata entry on the track format. Any real
+ * recording carries some — a creation timestamp is enough — so the export dies partway through:
+ *
+ * ```
+ * Caused by: java.lang.UnsupportedOperationException
+ *     at androidx.media3.muxer.OggMuxer.addMetadataEntry(OggMuxer.java:123)
+ *     at androidx.media3.transformer.MuxerWrapper.addTrackFormat(MuxerWrapper.java:488)
+ * ```
+ *
+ * They are standalone muxers, not Transformer-compatible ones. WAV fails a second way even before
+ * that: `DefaultEncoderFactory` has no PCM encoder, so Transformer reports "No MIME type is
+ * supported by both encoder and muxer" rather than passing raw samples through.
+ *
+ * Both were observed on a CI API 35 emulator, not inferred — the tests that found them were written
+ * on the assumption these containers worked.
+ *
+ * So Media3 writes MP4 and nothing else. WebM, Ogg, WAV and raw AAC belong to FFmpeg, which already
+ * produces all of them and has instrumented coverage asserting the produced files.
+ */
+@UnstableApi
+object Media3Muxers {
+
+    /**
+     * The factory for [container], or null when Media3 cannot write it.
+     *
+     * A null here must agree with [org.libremediaconverter.model.ConversionRouter]'s container set;
+     * `Media3MuxersTest` asserts they do. They drifted once already, and expensively: the router
+     * claimed five containers while the engine silently wrote MP4 for all of them.
+     */
+    fun factoryFor(container: Container): Muxer.Factory? = when (container) {
+        // Transformer's own default. Named explicitly so the engine states its container rather
+        // than inheriting one, which is how the MP4-for-everything bug went unnoticed.
+        Container.MP4 -> DefaultMuxer.Factory()
+
+        Container.WEBM,
+        Container.OGG,
+        Container.WAV,
+        Container.AAC_ADTS,
+        Container.MKV,
+        Container.MP3,
+        Container.GIF,
+        Container.IMAGE_SEQUENCE,
+        -> null
+    }
+}
diff --git a/app/src/main/java/org/libremediaconverter/convert/Transcoders.kt b/app/src/main/java/org/libremediaconverter/convert/Transcoders.kt
index c2594db..ac284d1 100644
--- a/app/src/main/java/org/libremediaconverter/convert/Transcoders.kt
+++ b/app/src/main/java/org/libremediaconverter/convert/Transcoders.kt
@@ -5,15 +5,23 @@ import android.net.Uri
 import org.libremediaconverter.ffmpeg.FFmpegEngine
 import org.libremediaconverter.model.ConversionRequest
 import org.libremediaconverter.model.DeviceCodecs
+import org.libremediaconverter.model.OutputFormat
 import org.libremediaconverter.codec.AndroidDeviceCodecs
 import java.io.File
 
 /** The hardware conversion path. Implemented by [Media3Engine]. */
 interface HardwareTranscoder : AutoCloseable {
+    /**
+     * Takes the whole [OutputFormat] rather than just a video MIME type.
+     *
+     * The narrower signature was the reason "extract audio to M4A" produced an HEVC video track:
+     * the container, the audio codec and "this output has no video at all" had nowhere to travel,
+     * so the engine defaulted all three.
+     */
     suspend fun transcode(
         input: Uri,
         output: File,
-        videoMimeType: String = androidx.media3.common.MimeTypes.VIDEO_H265,
+        format: OutputFormat = OutputFormat.MP4_H265,
         onProgress: (Int) -> Unit = {},
     )
 }
diff --git a/app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt b/app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt
index 1d56210..d23616b 100644
--- a/app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt
+++ b/app/src/main/java/org/libremediaconverter/model/ConversionRouter.kt
@@ -13,14 +13,19 @@ package org.libremediaconverter.model
  */
 object ConversionRouter {
 
-    /** Containers Media3 can mux. Anything else has to go to FFmpeg. */
-    private val MEDIA3_CONTAINERS = setOf(
-        Container.MP4,
-        Container.WEBM,
-        Container.OGG,
-        Container.WAV,
-        Container.AAC_ADTS,
-    )
+    /**
+     * Containers Media3 can mux. Anything else has to go to FFmpeg.
+     *
+     * MP4 alone. This set used to name WebM, Ogg, WAV and AAC-ADTS as well, on the strength of
+     * `media3-muxer` shipping a muxer for each — but none of those four can be driven by
+     * Transformer at all, for the reasons `Media3Muxers` records. Nothing caught it because the
+     * engine ignored the container entirely and wrote MP4 regardless, so the set being wrong and
+     * the engine being wrong cancelled out.
+     *
+     * Not private: `Media3Muxers` has to supply a `Muxer.Factory` for every entry, and a test
+     * asserts the two agree.
+     */
+    internal val MEDIA3_CONTAINERS = setOf(Container.MP4)
 
     /** WebM is codec-restricted: Media3's WebmMuxer writes only these. */
     private val WEBM_AUDIO = setOf(AudioCodec.OPUS, AudioCodec.VORBIS)
diff --git a/app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt b/app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt
index 6ae0507..f05a577 100644
--- a/app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt
+++ b/app/src/main/java/org/libremediaconverter/work/ConversionWorker.kt
@@ -3,7 +3,6 @@ package org.libremediaconverter.work
 import android.content.Context
 import android.net.Uri
 import android.util.Log
-import androidx.media3.common.MimeTypes
 import androidx.media3.common.util.UnstableApi
 import androidx.work.CoroutineWorker
 import androidx.work.Data
@@ -21,7 +20,6 @@ import org.libremediaconverter.model.Engine
 import org.libremediaconverter.model.EnginePreference
 import org.libremediaconverter.model.OutputFormat
 import org.libremediaconverter.model.QualityTier
-import org.libremediaconverter.model.VideoCodec
 import java.io.File
 
 /**
@@ -115,7 +113,7 @@ class ConversionWorker(
     ) {
         val engine = ConversionDependencies.hardware(applicationContext)
         try {
-            engine.transcode(inputUri, staged, media3MimeType(request.format)) { percent ->
+            engine.transcode(inputUri, staged, request.format) { percent ->
                 publishProgress(displayName, percent)
             }
             return
@@ -189,11 +187,6 @@ class ConversionWorker(
             }
         }
 
-    private fun media3MimeType(format: OutputFormat): String = when (format.videoCodec) {
-        VideoCodec.H264 -> MimeTypes.VIDEO_H264
-        else -> MimeTypes.VIDEO_H265
-    }
-
     override suspend fun getForegroundInfo(): ForegroundInfo =
         foregroundInfo(
             inputData.getString(KEY_DISPLAY_NAME) ?: "input",
diff --git a/app/src/test/java/org/libremediaconverter/convert/Media3MuxersTest.kt b/app/src/test/java/org/libremediaconverter/convert/Media3MuxersTest.kt
new file mode 100644
index 0000000..fed5dfd
--- /dev/null
+++ b/app/src/test/java/org/libremediaconverter/convert/Media3MuxersTest.kt
@@ -0,0 +1,60 @@
+package org.libremediaconverter.convert
+
+import androidx.media3.common.util.UnstableApi
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
+import org.junit.Test
+import org.libremediaconverter.model.Container
+import org.libremediaconverter.model.ConversionRouter
+
+/**
+ * Guards the agreement between the router's container set and the muxers that back it.
+ *
+ * These two drifted once already, and the failure was invisible in both directions: the router
+ * claimed WebM, Ogg, WAV and AAC-ADTS for Media3 — none of which Transformer can actually write —
+ * while [Media3Engine] ignored the container and produced MP4 for all of them. Two bugs that
+ * cancelled out, so nothing failed and the file was simply the wrong container.
+ *
+ * A JVM test rather than an instrumented one: constructing a factory touches no Android APIs, only
+ * `create()` does, so the mapping is checkable without a device.
+ */
+@UnstableApi
+class Media3MuxersTest {
+
+    @Test
+    fun `every container the router sends to Media3 has a muxer factory`() {
+        ConversionRouter.MEDIA3_CONTAINERS.forEach { container ->
+            assertNotNull(
+                "$container is routed to Media3 but has no Muxer.Factory",
+                Media3Muxers.factoryFor(container),
+            )
+        }
+    }
+
+    @Test
+    fun `containers the router withholds from Media3 have no factory`() {
+        Container.entries
+            .filter { it !in ConversionRouter.MEDIA3_CONTAINERS }
+            .forEach { container ->
+                assertNull(
+                    "$container has a Muxer.Factory but the router never sends it to Media3",
+                    Media3Muxers.factoryFor(container),
+                )
+            }
+    }
+
+    /**
+     * MP4 is the whole of it.
+     *
+     * Pinned as a value rather than left implicit, because `media3-muxer` shipping a `WebmMuxer`,
+     * `OggMuxer`, `WavMuxer` and `AacMuxer` makes it look like there should be five. Those four
+     * throw `UnsupportedOperationException` from `addMetadataEntry`, which `MuxerWrapper` calls
+     * unconditionally — see [Media3Muxers]. If a future Media3 release fixes that, this test is
+     * where the decision to widen the set gets made deliberately.
+     */
+    @Test
+    fun `Media3 can write MP4 and nothing else`() {
+        assertEquals(setOf(Container.MP4), ConversionRouter.MEDIA3_CONTAINERS)
+    }
+}
diff --git a/app/src/test/java/org/libremediaconverter/model/ConversionRouterTest.kt b/app/src/test/java/org/libremediaconverter/model/ConversionRouterTest.kt
index 36ab736..a00432b 100644
--- a/app/src/test/java/org/libremediaconverter/model/ConversionRouterTest.kt
+++ b/app/src/test/java/org/libremediaconverter/model/ConversionRouterTest.kt
@@ -39,13 +39,16 @@ class ConversionRouterTest {
     }
 
     @Test
-    fun `webm vp9 routes to ffmpeg because media3 cannot encode vp9`() {
-        // Transformer.setVideoMimeType accepts only H.263/H.264/H.265/MP4V. The WebM
-        // muxer exists, but there is no VP9 *encoder* behind it, so producing WebM
-        // video is FFmpeg's job even though the container is nominally supported.
+    fun `webm vp9 routes to ffmpeg`() {
+        // Two independent reasons, either of which is sufficient: Transformer.setVideoMimeType
+        // accepts only H.263/H.264/H.265/MP4V, so there is no VP9 encoder — and Media3 cannot mux
+        // WebM at all, which it was previously credited with. The container check runs first and
+        // is the more fundamental of the two: even given a VP9 encoder, the file could not be
+        // written. This asserted NO_PLATFORM_ENCODER while the container was wrongly believed to
+        // be supported.
         val d = route(OutputFormat.WEBM_VP9)
         assertEquals(Engine.FFMPEG, d.engine)
-        assertEquals(Reason.NO_PLATFORM_ENCODER, d.reason)
+        assertEquals(Reason.CONTAINER_UNSUPPORTED, d.reason)
     }
 
     @Test
@@ -173,6 +176,31 @@ class ConversionRouterTest {
         assertEquals(Reason.USER_FORCED_SOFTWARE, d.reason)
     }
 
+    // --- containers Media3 cannot write -------------------------------------
+
+    /**
+     * WAV, Opus and raw AAC used to be claimed for Media3 and are not any more.
+     *
+     * `media3-muxer` ships a muxer for each, which is why they were listed — but none can be driven
+     * by Transformer, so the export dies at the muxer. They were never actually produced on the
+     * hardware path: the engine ignored the container and wrote MP4 into a file named `.wav`.
+     * FFmpeg produces all three, and `FFmpegEngineTest` asserts the produced files.
+     */
+    @Test
+    fun `audio containers Media3 cannot mux route to FFmpeg`() {
+        listOf(OutputFormat.WAV, OutputFormat.OPUS).forEach { format ->
+            val d = route(format)
+            assertEquals("$format should need FFmpeg", Engine.FFMPEG, d.engine)
+            assertEquals(Reason.CONTAINER_UNSUPPORTED, d.reason)
+        }
+    }
+
+    /** M4A is the audio format that does stay on hardware, because its container is MP4. */
+    @Test
+    fun `m4a stays on hardware because MP4 is a container Media3 can write`() {
+        assertEquals(Engine.MEDIA3, route(OutputFormat.M4A_AAC).engine)
+    }
+
     // --- exhaustiveness ----------------------------------------------------
 
     @Test