The plugin is outstanding and easy to use, and will likely be the jumping off point for a whole bunch of other outstanding soundtrack mods.
That said, I was highly dubious of whether M2 would benefit from a soundtrack, but from the moment Mark Summer's music kicks in on Waterloo, you can feel it. Not all the tracks are my favorite but you'll probably really like the direction of at least one of the composers here. Some of the compositions absolutely elevate some of my least favorite levels into truly memorable experiences. Definitely worth playing through with this at least once.
This plugin enables M2SE music with the stock Marathon 2 or Marathon 2 for Windows 95 map.
Requires Aleph One 1.7b1. Simply unzip and copy the folder to your Marathon 2 Plugins directory.
Credits from the M2-SE Music Readme (included):