What goals are you trying to accomplish? If you are trying to use MP3, that will be difficult as the music files are in a different format.
But if you want to, say replace Elite 4 with Champion battle music, then just follow what this link tells you to do:
http://projectpokemon.org/forums/showthread.php?21005-B-W-Champion-Music
You can also use Tinke to modify the SDAT file. Extract the SDAT, open it with Tinke, and move things around. Be weary that the files come in three parts and you need to replace all three parts if you plan on moving music around. Just be sure to check up on the names.