When flicking through youtube vids I happened to come across this one which (
http://uk.youtube.com/watch?v=YsiSLX2UEVc) gives a walkthrough of how to put your own music in the background of brawl. It's surprisingly easy to do and gives much more freedom in terms of soundtrack. On top of that it works with any game in which you can ajust the music/sound effect volume. If you get stuck for choices of music to put in here are a few of my recommendations:
Ganondorf Sword Battle (LoZ:TP) - Bridge of Eldin
Normal Battle (Pkmn Colosseum) - Pokemon Stadium
Melty Molten Galaxy (SMG) - Norfair
Champion of Destruction (Super Paper Mario) - Halberd
Ghostbusters - Luigi's Mansion
etc...