Problem is usually wind roar. Look for a camera that has a remote mic input, I know the GoPros have, but there will be others. Usually the mic needs to be out of the wind, maybe tucked in the fairing somewhere or in a backpack None of the small video cams have very good sound recording, really if you want really good quality sound your going to probably have to use a separate sound recorder. Then comes the challenge of synchronizing sound and vision
Gopro may have but then you can't use them in the sealed clear plastic case so very vulnerable. But it's probably the only way to get decent sound by mounting the mic facing backwards to stop it picking up wind noise but still record the exhausts sound.
I ended up picking up a Gopro2 with loads of bits on fleabay for £100, bought a mic for £4.95 and drilled one of the housing units to connect it, it is still protected, the wire is run through the bikes plastics to just behind the frame above the Yoshi end can. Just the job clear exhaust/engine note and no wind noise. Cant wait for some tunnel action...
