fix(plex): persist transcode audio stream switches

close #1326

lib/screens/video_player/parts/episode_navigation.dart

@@ -150,10 +150,11 @@ extension _VideoPlayerEpisodeNavigationMethods on VideoPlayerScreenState {
     // Read the client before any await — context across an async gap. A
     // missing client leaves this null and the guard below reports it.
     final serverId = _currentMetadata.serverId;
-    PlexClient? subtitleClient;
-    if (isSubtitleChange && serverId != null) {
+    final isPlexBacked = _currentMetadata.backend == MediaBackend.plex;
+    PlexClient? streamSelectClient;
+    if ((isSubtitleChange || (isAudioChange && isPlexBacked)) && serverId != null) {
       try {
-        subtitleClient = context.getPlexClientForServer(ServerId(serverId));
+        streamSelectClient = context.getPlexClientForServer(ServerId(serverId));
       } catch (_) {}
     }
 
@@ -162,18 +163,19 @@ extension _VideoPlayerEpisodeNavigationMethods on VideoPlayerScreenState {
         await saveMediaVersionIndexFor(_currentMetadata, effectiveMediaIndex);
       }
 
-      if (isSubtitleChange) {
+      if (isSubtitleChange || (isAudioChange && isPlexBacked)) {
         final partId = _currentMediaInfo?.partId;
-        if (subtitleClient == null || partId == null || effectiveSubtitleStreamId == null) {
-          throw StateError('No Plex part available for subtitle stream selection');
+        if (streamSelectClient == null || partId == null) {
+          throw StateError('No Plex part available for stream selection');
         }
-        final saved = await subtitleClient.selectStreams(
+        final saved = await streamSelectClient.selectStreams(
           partId,
-          subtitleStreamID: effectiveSubtitleStreamId,
+          audioStreamID: isAudioChange ? effectiveAudioStreamId : null,
+          subtitleStreamID: isSubtitleChange ? effectiveSubtitleStreamId : null,
           allParts: true,
         );
         if (!saved) {
-          throw StateError('Failed to select subtitle stream');
+          throw StateError('Failed to select streams');
         }
       }
 

lib/services/plex_client.dart

@@ -1237,6 +1237,7 @@ class PlexClient
       if (queryParams.isEmpty) {
         return true;
       }
+      queryParams['allParts'] = 1;
 
       // Use PUT request on /library/parts/{partId}
       return _wrapBoolApiCall(

test/services/plex_playback_data_request_test.dart

@@ -56,6 +56,24 @@ void main() {
     return client.buildTranscodeSidecarSubtitlesForTesting(mediaInfoWithSubtitles(subtitleTracks));
   }
 
+  test('selectStreams sends audio stream selection with allParts', () async {
+    final requests = <http.Request>[];
+    final client = makeClient((request) async {
+      requests.add(request);
+      return http.Response('', 200);
+    });
+    addTearDown(client.close);
+
+    final saved = await client.selectStreams(99, audioStreamID: 301, allParts: true);
+
+    expect(saved, isTrue);
+    expect(requests, hasLength(1));
+    expect(requests.single.method, 'PUT');
+    expect(requests.single.url.path, '/library/parts/99');
+    expect(requests.single.url.queryParameters['audioStreamID'], '301');
+    expect(requests.single.url.queryParameters['allParts'], '1');
+  });
+
   test('playback metadata request includes streams for transcode sidecar subtitles', () async {
     final requests = <Uri>[];
     final client = makeClient((request) async {