Fixed channel override not triggering multi-audio track MKV downloads.

Also adds proper `ValueError` handling in the channel overwrites
API view to return a structured 400 error response.

backend/channel/serializers.py

@@ -19,6 +19,7 @@ class ChannelOverwriteSerializer(
     audio_multistream = serializers.BooleanField(
         required=False, allow_null=True
     )
+    audio_languages = serializers.CharField(required=False, allow_null=True)
     autodelete_days = serializers.IntegerField(required=False, allow_null=True)
     index_playlists = serializers.BooleanField(required=False, allow_null=True)
     integrate_sponsorblock = serializers.BooleanField(

backend/channel/src/index.py

@@ -268,6 +268,7 @@ class YoutubeChannel(YouTubeItem):
             "download_format",
             "download_container",
             "audio_multistream",
+            "audio_languages",
             "autodelete_days",
             "index_playlists",
             "integrate_sponsorblock",

backend/channel/views.py

@@ -175,7 +175,11 @@ class ChannelApiView(ApiBaseView):
             )
 
         if overwrites := validated_data.get("channel_overwrites"):
-            channel_overwrites(channel_id, overwrites)
+            try:
+                channel_overwrites(channel_id, overwrites)
+            except ValueError as err:
+                error = ErrorResponseSerializer({"error": str(err)})
+                return Response(error.data, status=400)
             if overwrites.get("index_playlists"):
                 index_channel_playlists.delay(channel_id)
 

backend/download/src/yt_dlp_handler.py

@@ -391,6 +391,27 @@ class VideoDownloader(DownloaderBase):
             if lang.strip()
         ]
 
+    def _is_audio_multistream_enabled(self, channel_id: str) -> bool:
+        """return True if audio_multistream is enabled globally or via channel overwrite"""
+        overwrites = self.channel_overwrites.get(channel_id, {})
+        if "audio_multistream" in overwrites and overwrites["audio_multistream"] is not None:
+            return bool(overwrites["audio_multistream"])
+        return bool(self.config["downloads"].get("audio_multistream"))
+
+    @staticmethod
+    def _discover_audio_languages(formats: list[dict]) -> list[str]:
+        """collect all unique language codes from available audio formats"""
+        seen: set[str] = set()
+        langs: list[str] = []
+        for f in formats:
+            if (f.get("acodec") or "none") == "none":
+                continue
+            lang = (f.get("language") or "").strip()
+            if lang and lang not in seen:
+                seen.add(lang)
+                langs.append(lang)
+        return langs
+
     def _dl_single_vid(self, youtube_id: str, channel_id: str) -> bool:
         """download single video, with optional multi-language audio merging"""
         obs = self.obs.copy()
@@ -400,9 +421,20 @@ class VideoDownloader(DownloaderBase):
         languages = self._get_audio_languages(channel_id)
         hls_formats: dict[str, str] = {}  # lang -> format_id for HLS post-process
 
+        if not languages and self._is_audio_multistream_enabled(channel_id):
+            # audio_multistream is on but no explicit language list – auto-discover
+            print(f"{youtube_id}: audio_multistream enabled, auto-discovering languages")
+            formats = self._get_formats(youtube_id)
+            if formats:
+                languages = self._discover_audio_languages(formats)
+                print(f"{youtube_id}: discovered audio languages: {languages}")
+        else:
+            formats = None  # will be fetched below if needed
+
         if languages:
             print(f"{youtube_id}: applying audio languages {languages}")
-            formats = self._get_formats(youtube_id)
+            if formats is None:
+                formats = self._get_formats(youtube_id)
             if formats:
                 dash_fmt, hls_formats = self._resolve_audio_formats(
                     formats, languages

backend/video/src/index.py

@@ -342,12 +342,17 @@ class YoutubeVideo(YouTubeItem, YoutubeSubtitle):
         if channel_overwrites.get("download_container"):
             container = channel_overwrites.get("download_container")
 
-        # audio_languages forces mkv for multi-stream support
+        # audio_languages or audio_multistream forces mkv for multi-stream support
         audio_languages = (
             channel_overwrites.get("audio_languages")
             or self.config["downloads"].get("audio_languages")
         )
-        if audio_languages:
+        audio_multistream = (
+            channel_overwrites.get("audio_multistream")
+            if channel_overwrites.get("audio_multistream") is not None
+            else self.config["downloads"].get("audio_multistream")
+        )
+        if audio_languages or audio_multistream:
             container = "mkv"
 
         return container

frontend/src/pages/ChannelAbout.tsx

@@ -50,6 +50,7 @@ const ChannelAbout = () => {
   const [downloadContainer, setDownloadContainer] = useState<'mp4' | 'mkv' | null>(null);
   const [audioMultistream, setAudioMultistream] = useState<boolean | null>(null);
   const [audioMultistreamWarning, setAudioMultistreamWarning] = useState<string | null>(null);
+  const [audioLanguages, setAudioLanguages] = useState<string | null>(null);
   const [autoDeleteAfter, setAutoDeleteAfter] = useState<number | null>(null);
   const [indexPlaylists, setIndexPlaylists] = useState(false);
   const [enableSponsorblock, setEnableSponsorblock] = useState<boolean | null>(null);
@@ -76,6 +77,7 @@ const ChannelAbout = () => {
           channelResponseData?.channel_overwrites?.audio_multistream ?? null,
         );
         setAudioMultistreamWarning(null);
+        setAudioLanguages(channelResponseData?.channel_overwrites?.audio_languages ?? null);
         setAutoDeleteAfter(channelResponseData?.channel_overwrites?.autodelete_days ?? null);
         setIndexPlaylists(channelResponseData?.channel_overwrites?.index_playlists ?? false);
         setEnableSponsorblock(
@@ -356,6 +358,19 @@ const ChannelAbout = () => {
                   <p className="settings-error">{audioMultistreamWarning}</p>
                 )}
               </div>
+              <div className="settings-box-wrapper">
+                <div>
+                  <p>Audio Languages</p>
+                </div>
+                <InputConfig
+                  type="text"
+                  name="audio_languages"
+                  value={audioLanguages}
+                  setValue={setAudioLanguages}
+                  oldValue={channel.channel_overwrites?.audio_languages ?? null}
+                  updateCallback={handleUpdateConfig}
+                />
+              </div>
               <div className="settings-box-wrapper">
                 <div>
                   <p>

frontend/src/pages/Channels.tsx

@@ -22,6 +22,7 @@ type ChannelOverwritesType = {
   download_format: string | null;
   download_container: 'mp4' | 'mkv' | null;
   audio_multistream: boolean | null;
+  audio_languages: string | null;
   autodelete_days: number | null;
   index_playlists: boolean | null;
   integrate_sponsorblock: boolean | null;