if I remember correctly its mp3 format.
I tried using my BT audio, but it doesn't interact well with my GS3, so I having to revert to using a 32gb usb drive to hold all my music and play it that way.
not to thrilled about it currently until I find a decent replacement music playing app that will work with the head unit BT.
Congrats and welcome. I've not used that feature as I stream most of my music or use an iPod. Did you try the Manuel?