Do you want AM/FM/CD/Bluetooth or just Bluetooth? If just Bluetooth, I would suggest motorcycle speakers from BOSS. I think DF sells a set and you can find them on Amazon. If you want more bass for more money, Lethalcast just installed these ATV speakers with a sub. You can find more info in his thread located here: http://dfkitcar.com/forum/index.php?threads/lethalcasts-city-goblin-2009-ss-tc-g85.583/page-10#post-15710
+1 for helmet speakers/communicators. We used cardo scala riders for bike to bike/group talking and I used it a ton for calls, music, etc. I'll be reinstalling mine in the helmet I use for the goblin.If you truly want good sound and other features to boot, this helmet has built in audio and bluetooth, not to forget intelligent noise cancelling.
We are going to get a set of the Momentum Lites.
http://www.buysena.com/MOI-STD-MB-XS-01.html?sc=23&category=144878