I totally agree with charlie b. The SENA SMH10 has great sound. A couple of things to think about...
First, the speakers that come with the SENA SMH10 are tuned to work through earplugs. I found that with foam earplugs in the sound from the speakers is perfectly fine. Although the volume wasn't high enough for me.
Second, I highly recommend getting a pair of in-ear headphones. I recently purchased a pair from
http://www.plugup.com. You also have to buy the SENA headphone adapter, of course. But they work really well and mean I don't have to have separate earplugs. And now the sound volume is really excellent!