When the plugin's config request names OBS mixer tracks 2-6, the node asks for an AAC and an Opus encoder for the full mix (Track 1) and for up to two named tracks. The fmp4-direct ingest signs with muxl's SourceTrackMeta, so every segment's catalog carries each track's alternateGroup and label. Audio selection is group-aware: the lowest group is the main audio, and HLS, RTMP push, rendition playback, codec completion and the WebRTC mix all pick from it as before. Segments without groups keep today's meaning. Segment records list every audio track with its real codec and label. WebRTC playback adds one Opus track per stem group after the main audio, in order, so they land on the viewer's offered audio transceivers; transcoded renditions carry the stems too. Needs streamplace/muxl#8 (track labels); go.mod pins its branch commit until a release.