setAudioTrack method
Selects the audio track via a typed Track — Track.auto defers to mpv's automatic choice (container default or first audio track), Track.off disables audio output entirely, and Track.id selects a specific track by its mpv ID (match an entry in PlayerState.tracks).
State updates flow through the current-tracks/audio observer
(no optimistic update — mpv may reject an unknown id).
Implementation
Future<void> setAudioTrack(Track track) async {
_checkNotDisposed();
await _ready;
_prop('aid', track.mpvValue);
}