HOW TO ADD CUSTOM MUSIC USING THE AUDIO EDITOR:
Create a mod(Mod_Name)
put your final wav file in the:
\Mod Tools\Data Generic?\Mod_Name\Sound\Music\
folder
Open the audio editor, open the mod, expand to the music folder, select the wav file(s) you want to burn, and burn them, they will appear in the music folder in your mod. I dunno how to get them into the game, since I dont have any music yet to put into the game.
You cannot use MP3 files. They must be wav.
(Credits goes to Beroc 
http://forums.relicnews.com/member.php?userid=28079)