These tracks really add to the Sci Fi feel of the originals and I can see them as great music replacements for those wishing to play with updated graphics and sounds, but a few of the tracks seem to loose their subtleness such as tracks 6 & 14. Other than those nitpicks I think It's pretty dang good.
The music that is shipped with the Marathon release is based off of the classic Quicktime synth. This is the fed through fluidsynth which utilized a modern sample set — in this case, Don Allen's Timbres of Heaven. All tracks have been normalized and made working for in-game.
To install, simply replace the Music folder within the Marathon data folder