OK, I've successfully replaced the main menu theme song with a random music from another game. In this case the theme music from the game "Red Alert". The game uses .ogg files, but I can confirm .mp3 works too. To replace the opening music, or leader music, be sure your track has the exact same name as the track you want to replace. I don't know if replacing background music requires you to rename the songs or not, as I am too lazy to put in a lot of effort!
Oh, btw. Don't change Elizabeth's peace music. It's so awesome! Go listen to it now!