Can it be done?
GalCiv2 Forums
I saw that there is a music folder in the game files and it implies that the music that plays while you are playing can be customized.
Can it be customized?
Just put a track with the same filename as the one you want to replace in
My Documents\My Games\GC2TwilightArnor\Music (GC2DarkAvatar if you're playing Dark Avatar)
or Event Music if you want to change out the battle music, opening menu track, that sort of thing.
I found a few mp3 files with music of about 1:30 min long for the diplomacy window -- basically new race music -- and now with some new custom races I have some new background tunes while "talking to aliens". 
Would it be possible for me to give my custom race it's own theme music without replacing other music?
Never tried it. If it does work, you'll need to put your custom track in the Event Music folder (either in a mod folder or in the GC2TwilightArnor folder), then take the name of the track and edit it into your custom race in Notepad. For example:
<RaceMusic>CustomRaceTrack</RaceMusic>
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