for MI1, 2 3 + 4, MI1 and MI2 are played from an MT-32 Synth and recorded (with some re-mixing of the sound-levels) onto MP3s. The composers composed the music for those two (and other MIDI SCUMM games) using an MT-32, so it's as the composer envisioned the music, not replayed using crappy windows midi emulation, which really distorts and ruins the sound in my opinion.
you can try: http://www.s-island.mixnmojo.com
They have the original MT-32 Music without any re-mixing as OGG files, which you can play using VLC media player. (All they did in the first site I listed was change a few sounds, and add some reverb really).
Basically, if you want the Vanilla music, as oringally ripped from the original game, in best quality, go to the second one, otherwise if you don't mind them being a little 'remastered (slightly different sounds, different mixing) go to the first link)