Brewology

Not that I've heard of, no.
I tihnk you do need custom firmware in order to add the music.prx plugin.

That or just buy ATV: Offroad and you can play the game while listening to your own tracks =p

I think there's a way to modify the GTA ISO's to play your own songs.
But that's another part of hacking that not a lot of people on this site like to talk about.

Whats wrong with switching the songs out on the ISO?

Well you can't do that since you don't have cfw. Unless Sony adds support to do it, which would be great, you won't be able to do it on OFW.