I'm interested in this too.. My interest is mostly in the Williams system 4-7 games, and mostly from the perspective of building replacement sound/voice boards based on something like an arduino or RPi, rather than improving or changing the sounds. I dont know, but I assume that on the williams board, there's not much that would translate well to MIDI, but I could be wrong. This has always been kind of a back-burner "wouldnt it be cool" project for me but I haven't as much as looked at the schematics, but right now I actually have a Gorgar that needs a sound board ,and I have some sound boards to fix and install, so perhaps now is the time to learn and experiment with it!