I use an mp3 splitter to detach the intro from the rest of the file, and insert it into the main menu as an option to "play intro." If your DVD authoring software doesn't allow you to play musical features from the menu, you can also make a "blank screen" flash movie with the intro part of the mp3 playing, and put that blank screen video in as a menu option... achieves the same result, it's just that your screen goes black while the intro plays, instead of staying on the menu.