Hello Myreaux, did you file a bug report to Apple about this issue? If yes, did you hear back from them? I was about to report the same problem. In my experience it's not just isMuted though: moving/adding/deleting MIDI notes during playback through the AVMusicTrack API also triggers this bug. The only (hacky) workaround I have found is to manually implement looping by setting isLoopingEnabled to false and by resetting AVAudioSequencer.currentPositionInBeats when it exceeds the desired loop length inside a Timer callback.