Steve Berman (caver in FL) used to make a device called a Diveman which was a walkman in a sealed canister with earphones from underwater metal detectors. I can't find anything via google, but I didn't work at it long. It should be easy enough to put a NetMD minidisc player in a canister on random playing MP3s (cheap, runs on 1AA). The hard part would be the earphones. If you didn't mind mono, you could probably just get a replacement earphone from a metal detector retailer - stereo will be much harder. Like a camera though, you'll eventually flood it. Alternatively, you could make an underwater boom box by attaching the speakers to the inside of the canister so that they reverberate through the water. Downside would be that everyone else gets to hear it...
Let me know what you come up with, I'm interested to see if you find anything. Good luck.