My own personal Fiesta soundtrack
 

A.K.A: What I listen to while playing.

The in-game music's ok but sometimes it can feel like is has a mid-90's, low-budget " Power Rangers" type of vibe (that's what I think), so I plug into my iPod listen to whatever I've currently on my most played.

Most recently:
Kardinal Offishall: Dangerous; the other day while fighting werebears & goblin swordsmen. Yeah, I AM a bad girl.
Snow Patrol: What if The Storm Ends; (this is a third of a three segment, 16 minute song (The Lightning Strike) and my favorite of the three). It has sort of become the current theme of one of my characters (outside of Fiesta) who I've associated w/ storms. But I think it make a great fighting/battle song as well.
Joy Division: Transmission; I think the chorus describes it best. "Dance, dance, dance, dance, dance to the radio..."
Dire Straits: Expresso Love.
VHS or Beta: Burn it All Down.

I just crank up my winamp.

Same here. Sometimes I'll whittle down a playlist, but it's usually on random. Anything from the Shaggs to Irish folk music to spoken word/comedy.

I think theres a background music folder somewhere in all the fiesta files
you can just make a copy of your music files, rename them so they match the existing ones and replace it.
I don't know if this is against ToS or not but I've heard of people doing it in some other games.

Any manipulation of game files is against TOS. That's a hell of a lot of work, too.

I play my radio on my Psp with J-pop or Tencho

I like how Cabal has its own music player in-game where you can put your own music in.

Why bother? Just turn of the music volume in your settings, and play your music in your music player.