anyway, the name of the lumps that you use for music are named by the musicslot that they take, E.G gfz1 O_MAP01M, if you make a music wad that has that lump in it, you'll replace gfz1 music, there is also O_MAP02M and it gos all the way to O_MAP09M, there is also empty slots available, like O_MAP06M and O_MAP10M.
Later slots are named differently, super sonic music is O_SUPERS and invincible is O_INVINC, try to avoid using existing slots for new music.
http://www.sepwich.com/foxboy/Srb2World/text/newmapheaders.html
you can find a list there.
And to fix that error just reinstall SRB2, and make a backup of gamedata.dat, unless you want to lose all the secrets that you have unlocked.