MOTM

I used to use a program called "Sound Globs" ages ago. It was a DOS program
that addressed the MPU-401 (or compatible) directly, so I doubt it would
behave well under Windows, otherwise I'd drag it out and try it again.

The key to using these kinds of programs is lots of editing. I did some neat
pieces with Globs, but you kind of have to tinker with it for a long time to
get the right feel, then record an hour of MIDI data, then use your
sequencer to edit the hell out of it.