The contour in it's waterproof case has the same sound problem but I have found that I never needed the waterproof case even in the heaviest downpours, a little waterproof tape around the join where the lens rotates is enough. The GoPro also has the same sound problem but unfortunately it often needs to be in it's waterproof case as it is very open to water ingress.Yes, although the US page has the included case listed under the Features tab but not so the UK page, the case is listed as included in the UK Specifications tab, so I was wrong.
Looking at some of the early customer reviews on the US site for both the wifi and non-wifi versions, and having downloaded the manual, one annoying drawback seems to be that the fixed mount options all involve using the waterproof case, which seals the camera in, of course, and muffles the audio. However, one review claims the case muffles the audio, another says it makes little difference, so perhaps more reports needed before jumping to conclusions.
The helmet strap BLT-HB1 seems to use a different clamp arrangement that doesn't need the case (another headband mount VCT-GM1 does use the waterproof case, btw). It may be that a bar mount with decent audio would require some surgery on the helmet mount to clamp it to a bar fixing but I'll wait for some more user reviews to see what happens.
John
The Sony mounting does look disappointing especially if you want to top helmet mount it, looks as bad as the current GoPro.
I have ordered the Sony instead of the C+2 mainly because I think the changes from the C+ to the C+2 are trivial, apart from the higher bit rate.