I am trying to make a mod that changes the main menu theme, but I can't figure out what the associated music track is.

If I remember correctly the actual Music Track record is hardcoded. However the file is of course not, so you can alter it by placing the new music in the exact same folder location with the same name. This will overwrite the music packed up in the skyrim BSA with your new music.

EDIT: So name the file "mus_maintheme" and make sure it's an XWM, then place it in Data\music\special\

It didn't work. Skyrim stopped working, and closed itself out.

That means you have a missing masterfile. (CTD after opening logo)

What all did you do since it launched without issue last? Simply adding the file shouldn't be able to break Skyrim.