I can see where there is potential in this - individually, each instrument sounds better. But, like all attempts at throwing newer midi at the old tracks, they aren't working well together as a whole. Unbalanced, grainy, horribly distorted, and the normalisation would be better left off- it's painful to hear instruments suddenly get louder at a quiet point in a song. I definitely prefer the originals, or better yet Craig Hardgrove's versions. That said, I'd like to hear some of these tracks if you could nix the normalisation and maybe quiet the drums.
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