I wouldn't suggest using midi in your games. If the user is not using the microsoft synth then it will playback different on each system, depending upon the sound card they are using. If the user *is* using the microsoft synth, then it will playback pretty slugish and not leave much cpu time for your own processes. (at least on my system).
The best choice, I believe, would be to write the music to cd as audio tracks. But then you can't distribute the game over the internet with music.
There are alot of choices, and each one has it's own pro's and con's to it. I would experiment with each to find something that you are comfortable with.
Perhaps the others on this board could share their feelings on how they would (or do) incorperate a sound track into thier games.
you can try this ftp://www.xgames3d.com/games/gvaudio.zip it will play back mod files for you. I have also seen somewhere an public domain mp3 lib, but it is lost to me now.