The way I see MIDI working in godot would be to create another type of bus which handles MIDI data. It would probably work similarly to a sampler, by loading a sample library and assigning different sounds to corresponding MIDI messages. The user would be able to assign the midi messages to certain events in their game and when they get triggered the corresponding sample/sound will play. The sampler will essentially be creating something similar an audio bus for each sample and that bus will only play when a corresponding midi message is triggered. All of this will be contained in one track, which will also have all the signal processing features a standard audio bus.