I think the music file must be in the folder Data\Audio\Tracks. Then the file needs an entry in the music.ini like this:
Code:
MusicTrack Game_[YOURFACTION]_01
Filename = [YOURFACTION]_01.mp3
End
Now the music file is entered in the INIs but how to tell Generals to play it ingame? As far as I know, the music is called by a script.
Execute World Builder and open any official Generals map (System Maps). Goto "Edit" and search for "Scripts". In the new window click "Import Scripts" and select "SkirmishScripts.scb" located in Data\Scripts. You'll have to wait a second but then you see that at "PlyrCivilian" something has changed. Open the "directory tree" (don't know the correct english term actually) and you'll see that there are entries like "[ns A]USA Music" etc.
Now you can enter a new side and the music for it and after you finished everything export it.
I think you can just copy and paste most of the code from existing parts of the script. But make a copy of your "SkirmishScripts.scb" if something fails. Actually I never did this since I am at the "modelling, texturing and INI coding state" at the moment, music comes later.
Maybe it's just try and error but worth the trouble :-)
- HEXman