I've tried several means of getting audio to my helmet, from ear buds to in ear plug speakers at $300 a pop, but nothing has worked more elegantly than the XPro helmet speakers from Clearercom http://www.clearercom.com/xpro.htm at $80.
I've got a HJC AC12 which isn't the quietest helmet and at 140kph the tunes are clear and crisp with impressive bass response.
The speakers are extremely thin and fit comfortably in my lid attached with simple velcro with no issues of fumbling for ill fitting speaker buds in my earballs.
I don't like listening to tunes on my bike in the city or raging in the twisties but for flat out, mind numbing freeway riding, these babies are awesome!
I should also note that the guys at Clearercom are really upstanding people, who bent over backwards to make me happy, even over a small ticket item like the XPros.