There are some "My music fix" codes out there. They might help.
There's a thread in the help section where somebody had the same problem and the code is listed there.

My Music fix is only for if it freezes after you select a stage in My Music. This person's problem is it freezes when he tries to even get into My Music itself first.

Actually yesit freezes when I choose the Battlefield. Could someone provide a link to this code or give it to me straight instead of just telling me about it?

I was testing/changing the background music of brawl and it started to freeze every time I chose "My Music" even when I load brawl normally through the disc channel. It would make this loud BEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE-

I tried to change the Battlefield music (X04.brstm right?)

So i have to get rid of his Brawl- pac?

No.
FitKoopa.pac is the moveset, you can keep that.

Only FitKoopa00,01,02 etc. need Both .pac and .pcs to be the same

OH ok, lemme see if I got this straight...

So it should be like:

FitKoopa.pac
FitKoopa01.pac
FitKoopa01.pcs
etc?

Correct

Ok Thanks! I'll try it out now :p

One more question :O

I see some item texture hacks, where would I put those?

pf->system

OK THX!

So i have to get rid of his Brawl- pac?

No.
FitKoopa.pac is the moveset, you can keep that.

Only FitKoopa00,01,02 etc. need Both .pac and .pcs to be the same

OH ok, lemme see if I got this straight...

So it should be like:

FitKoopa.pac
FitKoopa01.pac
FitKoopa01.pcs
etc?

Correct

Ok Thanks! I'll try it out now :p

One more question :O

I see some item texture hacks, where would I put those?

So i have to get rid of his Brawl- pac?

No.
FitKoopa.pac is the moveset, you can keep that.

Only FitKoopa00,01,02 etc. need Both .pac and .pcs to be the same

OH ok, lemme see if I got this straight...

So it should be like:

FitKoopa.pac
FitKoopa01.pac
FitKoopa01.pcs
etc?

So i have to get rid of his Brawl- pac?

I downloaded some pretty awesome texture hacks for Bowser and they don't seem to work.

I tried every name i can think of for the textures:

FitBowser.pcs
Fitbowser.pcs
fitbowser.pcs
fitBowser.pcs
FitKoopa.pcs
Fitkoopa.pcs
etc.

Any advice? Oh I have Brawl minus btw.