You can find a good tutorial about how to burn DC games here :
http://www.megagames.com/dc/dc_backup_faq.shtml
Now, to change MvC2 musics, if they are CDDA, it's quite easy : just change the 1st audio tracks. But if they're sound files (I'd bet it's this case), you'll have to change these files, so you'll have to know their format and to make your own musics in this format.
Good luck.
