I was able to confirm that the callback executes in the code you posted. However, it was not what I had expected. I have confirmed that MusicPlayer plays MIDI files correctly. I want to know the MIDI data being output to the MIDI device in sync with the timing of the MIDI file playback. For example, this is used when you want to receive the MIDI data being output via a callback or similar method and display it on the GUI. In that case, what kind of callback or method is available?