You could always try Coreplayer, and see how you like that.
Or....Pocket Player...
http://www.conduits.com/products/player/
Or....Pocket Tunes...
http://www.pocket-tunes.com/
I used to use Ptunes on my Palm
Personally, I have not even played a song yet from the device, so I cant give my personal opinion on each one. I'm only giving what you could use.