Just got a pair of iASUS Helmet Speakers (http://www.iasus-concepts.com/nt/xsound.htm). My ears are hypersensitive to pressure (I can't use active noise reduction headphones or even wear a Schuberth helmet that I own because of the overpressure on my ears), so earbuds were out of the question. I couldn't keep regular Ipod earphones in while putting on my helmet. So the iASUS were sort of a last resort.
Here's the skinny: they are so thin that they fit very well even in my heavily padded Arai helmet. They connect with velcro, either to an included adhesive piece of female velcro, or directly to the lining. The sound is good. Not great, but heck, these are $50 helmet speakers, so I wasn't expecting anything audiophile.
The only problem is that they're not quite loud enough. I've ordered a $10 amplifier (http://www.amazon.com/Fiio-E3-Headphone ... A/ref=pd_c) and think that will solve the problem. Overall, I'd recommend the speakers