diff --git a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt
index ed4597b5..a1fa9283 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt
@@ -542,7 +542,12 @@ object NativeDownloadFinalizer {
                 if (successPath != null) break
             }
 
-            val decryptedPath = successPath ?: throw IllegalStateException("decrypt failed: $lastOutput")
+            val rawDecryptedPath = successPath ?: throw IllegalStateException("decrypt failed: $lastOutput")
+            val decryptedPath = normalizeDecryptedIsoBmffAudioPath(
+                rawDecryptedPath,
+                state,
+                shouldCancel,
+            )
             replaceStatePath(context, input, state, decryptedPath, deleteOld = true)
         } finally {
             if (successPath == null) {
@@ -554,6 +559,37 @@ object NativeDownloadFinalizer {
         }
     }
 
+    private fun normalizeDecryptedIsoBmffAudioPath(
+        path: String,
+        state: FinalizeState,
+        shouldCancel: () -> Boolean,
+    ): String {
+        if (!isMP4ContainerFile(path)) return path
+
+        val probedCodec = probePrimaryAudioCodec(path, shouldCancel)
+        val codec = normalizeAudioCodec(probedCodec.ifBlank { state.audioCodec.orEmpty() })
+        val desiredExt = NativeFinalizationPolicy.isoBmffAudioExtension(codec)
+        state.audioCodec = codec
+        if (path.lowercase(Locale.ROOT).endsWith(desiredExt)) return path
+
+        val target = File(buildOutputPath(path, desiredExt))
+        if (target.exists()) {
+            Log.w(TAG, "Cannot normalize ISO-BMFF audio extension; ${target.name} already exists")
+            return path
+        }
+        val source = File(path)
+        if (!source.renameTo(target)) {
+            Log.w(TAG, "Failed to normalize ISO-BMFF audio extension to ${target.name}")
+            return path
+        }
+        Log.i(
+            TAG,
+            "ISO-BMFF audio renamed: ${source.name} -> ${target.name} " +
+                "(codec=${codec.orEmpty().ifBlank { "unknown" }})",
+        )
+        return target.absolutePath
+    }
+
     private fun finalizeHighConversion(
         context: Context,
         input: FinalizeInput,
@@ -814,10 +850,10 @@ object NativeDownloadFinalizer {
         if (!currentFile.name.lowercase(Locale.ROOT).endsWith(".flac")) return
 
         val newExt = when {
-            codec == "aac" && isMP4ContainerFile(localInput) -> ".m4a"
+            isMP4ContainerFile(localInput) ->
+                NativeFinalizationPolicy.isoBmffAudioExtension(codec)
             codec == "mp3" -> ".mp3"
             codec == "opus" -> ".opus"
-            isMP4ContainerFile(localInput) -> ".m4a"
             else -> return
         }
         val renamed = File(
diff --git a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt
index f3dfb4c0..1c930178 100644
--- a/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt
+++ b/android/app/src/main/kotlin/com/zarz/spotiflac/NativeFinalizationPolicy.kt
@@ -68,6 +68,15 @@ internal object NativeFinalizationPolicy {
         }
     }
 
+    /**
+     * Audio-only ISO-BMFF files conventionally use .m4a. AC-4 remains .mp4
+     * because its passthrough and standards-repair path depends on that
+     * container identity.
+     */
+    fun isoBmffAudioExtension(audioCodec: String?): String {
+        return if (normalizeAudioCodec(audioCodec) == "ac4") ".mp4" else ".m4a"
+    }
+
     fun audioFormatForCodec(codec: String?): String? {
         return when (normalizeAudioCodec(codec)) {
             "flac" -> "FLAC"
diff --git a/android/app/src/test/kotlin/com/zarz/spotiflac/NativeFinalizationPolicyTest.kt b/android/app/src/test/kotlin/com/zarz/spotiflac/NativeFinalizationPolicyTest.kt
index e09fdb0b..cf2bc816 100644
--- a/android/app/src/test/kotlin/com/zarz/spotiflac/NativeFinalizationPolicyTest.kt
+++ b/android/app/src/test/kotlin/com/zarz/spotiflac/NativeFinalizationPolicyTest.kt
@@ -76,6 +76,26 @@ class NativeFinalizationPolicyTest {
         assertFalse(NativeFinalizationPolicy.isLosslessAudioCodec("aac"))
     }
 
+    @Test
+    fun isoBmffAudioUsesM4aExceptForAc4Passthrough() {
+        assertEquals(
+            ".m4a",
+            NativeFinalizationPolicy.isoBmffAudioExtension("opus"),
+        )
+        assertEquals(
+            ".m4a",
+            NativeFinalizationPolicy.isoBmffAudioExtension("ec-3"),
+        )
+        assertEquals(
+            ".m4a",
+            NativeFinalizationPolicy.isoBmffAudioExtension("aac"),
+        )
+        assertEquals(
+            ".mp4",
+            NativeFinalizationPolicy.isoBmffAudioExtension("ac-4"),
+        )
+    }
+
     @Test
     fun displayQualityUsesMeasuredLosslessSpecifications() {
         assertEquals(
diff --git a/lib/providers/download_queue_provider_finalization.dart b/lib/providers/download_queue_provider_finalization.dart
index 038f8344..df16c0be 100644
--- a/lib/providers/download_queue_provider_finalization.dart
+++ b/lib/providers/download_queue_provider_finalization.dart
@@ -44,6 +44,44 @@ bool _isMp4Container(String path) {
   return lower.endsWith('.m4a') || lower.endsWith('.mp4');
 }
 
+Future<String> _normalizeDecryptedIsoBmffAudioPath(
+  String path,
+  Map<String, dynamic> result,
+) async {
+  if (!_isMp4Container(path)) return path;
+
+  final probedCodec = await FFmpegService.probePrimaryAudioCodec(path);
+  final reportedCodec =
+      result['audio_codec']?.toString() ??
+      result['actual_audio_codec']?.toString() ??
+      result['format']?.toString();
+  final desiredExt = isoBmffAudioExtensionForCodec(
+    probedCodec ?? reportedCodec,
+  );
+  if (path.toLowerCase().endsWith(desiredExt)) return path;
+
+  final targetPath = path.replaceFirst(
+    RegExp(r'\.(?:m4a|mp4)$', caseSensitive: false),
+    desiredExt,
+  );
+  if (targetPath == path) return path;
+  try {
+    final target = File(targetPath);
+    if (await target.exists()) {
+      _log.w(
+        'Cannot normalize ISO-BMFF audio extension; target already exists: '
+        '$targetPath',
+      );
+      return path;
+    }
+    final renamed = await File(path).rename(targetPath);
+    return renamed.path;
+  } catch (e) {
+    _log.w('Failed to normalize ISO-BMFF audio extension for $path: $e');
+    return path;
+  }
+}
+
 extension _DownloadQueueFinalization on DownloadQueueNotifier {
   Future<_AutoConversionOutcome> _autoConvertDownloadedFile({
     required String itemId,
@@ -770,16 +808,24 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
         relativeDir: safRelativeDir,
         op: (tempPath, addCleanup) async {
           opStarted = true;
-          final decryptedTempPath = await FFmpegService.decryptWithDescriptor(
-            inputPath: tempPath,
-            descriptor: descriptor,
-            deleteOriginal: false,
-          );
-          if (decryptedTempPath == null) {
+          final rawDecryptedTempPath =
+              await FFmpegService.decryptWithDescriptor(
+                inputPath: tempPath,
+                descriptor: descriptor,
+                deleteOriginal: false,
+              );
+          if (rawDecryptedTempPath == null) {
             failStage = DownloadQueueNotifier._decryptStageDecrypt;
             return null;
           }
-          addCleanup(decryptedTempPath);
+          addCleanup(rawDecryptedTempPath);
+          final decryptedTempPath = await _normalizeDecryptedIsoBmffAudioPath(
+            rawDecryptedTempPath,
+            result,
+          );
+          if (decryptedTempPath != rawDecryptedTempPath) {
+            addCleanup(decryptedTempPath);
+          }
           if (repairAc4 && _isMp4Container(decryptedTempPath)) {
             try {
               await PlatformBridge.ensureAC4Config(decryptedTempPath, tempPath);
@@ -814,12 +860,12 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
     }
 
     if (repairAc4) {
-      final decryptedPath = await FFmpegService.decryptWithDescriptor(
+      final rawDecryptedPath = await FFmpegService.decryptWithDescriptor(
         inputPath: filePath,
         descriptor: descriptor,
         deleteOriginal: false,
       );
-      if (decryptedPath == null) {
+      if (rawDecryptedPath == null) {
         try {
           await deleteFile(filePath);
         } catch (_) {}
@@ -828,6 +874,10 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
           failStage: DownloadQueueNotifier._decryptStageDecrypt,
         );
       }
+      final decryptedPath = await _normalizeDecryptedIsoBmffAudioPath(
+        rawDecryptedPath,
+        result,
+      );
       if (_isMp4Container(decryptedPath)) {
         try {
           await PlatformBridge.ensureAC4Config(decryptedPath, filePath);
@@ -841,11 +891,14 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
       return _DecryptOutcome(decryptedPath);
     }
 
-    final decryptedPath = await FFmpegService.decryptWithDescriptor(
+    final rawDecryptedPath = await FFmpegService.decryptWithDescriptor(
       inputPath: filePath,
       descriptor: descriptor,
       deleteOriginal: true,
     );
+    final decryptedPath = rawDecryptedPath == null
+        ? null
+        : await _normalizeDecryptedIsoBmffAudioPath(rawDecryptedPath, result);
     return _DecryptOutcome(
       decryptedPath,
       failStage: decryptedPath == null
diff --git a/lib/utils/audio_format_utils.dart b/lib/utils/audio_format_utils.dart
index c7395b75..0bd44c94 100644
--- a/lib/utils/audio_format_utils.dart
+++ b/lib/utils/audio_format_utils.dart
@@ -118,6 +118,14 @@ String? normalizeAudioFormatValue(String? value) {
   };
 }
 
+/// Chooses the user-facing extension for an audio-only ISO-BMFF container.
+/// AC-4 stays `.mp4` because its compatibility repair and passthrough path
+/// depend on that container identity; other audio codecs use the conventional
+/// `.m4a` extension, including Opus and E-AC-3 streams carried inside MP4.
+String isoBmffAudioExtensionForCodec(String? codec) {
+  return normalizeAudioFormatValue(codec) == 'ac4' ? '.mp4' : '.m4a';
+}
+
 /// Resolves the actual audio codec reported by native metadata probing, while
 /// falling back to the container format when the codec is absent or generic.
 ///
diff --git a/test/models_and_utils_test.dart b/test/models_and_utils_test.dart
index de89cd28..8e16a36e 100644
--- a/test/models_and_utils_test.dart
+++ b/test/models_and_utils_test.dart
@@ -1038,6 +1038,13 @@ void main() {
       );
     });
 
+    test('uses m4a for ISO-BMFF audio except AC-4 passthrough', () {
+      expect(isoBmffAudioExtensionForCodec('opus'), '.m4a');
+      expect(isoBmffAudioExtensionForCodec('ec-3'), '.m4a');
+      expect(isoBmffAudioExtensionForCodec('aac'), '.m4a');
+      expect(isoBmffAudioExtensionForCodec('ac-4'), '.mp4');
+    });
+
     test(
       'detects Dolby formats from stored scan format before file extension',
       () {