If you wear a Shred Ready helmet, I'd check out the Can-Can ear flaps.
www.diamondbrand.com/commerce/catalog.php?id=1815
H20 audio seems to be the benchmark for waterproof mp3 players as well.
www.h2oaudio.com/
Gath helmets have an audio system as well, but I have not heard enough reviews to recommend them (although I may get something like that at some point soon, for communication while riverboarding).