After reading the introduction to audio formats article here on gamedev.net. I had the notion that midi could help me with a couple of things. I'm a lone wolf developer yet I not only want music for my games, and not only mood-specific music, but unscripted mood-specific music. What I'm hoping is that I can use midi to compose the track according to the situation rather than trigger a song according to a script or simple variables. Any advice on this with or without midi would be appreciated. Second, can suitable Instrument Definition Files be made to replicate human voice phonemes? I'm doing text to speech as well, one way or another. How easy is it to alter the IDF's? I'd like to be able to make multiple voices or at least multiple tones from one set of files.

Look into DirectMusic (part of DirectX). I believe it has features for doing this.

I believe you can do this with MOD music as well.

MIDI isn't really designed for it, but it's possible.