First thing is to get familiar with how to talk to MPU401 and look at the MIDI standard. Sending MIDI bytes via MPU401 is quite simple, rest is then folowing MIDI standards. MT32 support is then just sending right insturment numbers and whatnot, subset of General MIDI data.

Not sure off the top of my head, but any of the MT-32 specific features will probably be in the MIDI implementation guide. The MT-32 is a little more programmable than a ROMpler, so it can be worth learning if you want a soundtrack that takes advantage of all it has to offer.

it has 2 or 3 IO ports and you just shove bytes at it, in dumb MPU 401 mode.

http://www.piclist.com/techref/io/serial/midi/mpu.html