Mainly just figured I'd throw this tidbit into the ring...as I don't think USB is the problem.
I'm (now) inclined to agree.
I spent the weekend slowly transplanting the driver files that were listed from Windows 7 into Windows 8. This involved some fun with the command line, and also disabling driver signature enforcement.
Whilst the mic still worked with the windows 7 driver files in place, it still had the same issue.
I need to find a microphone that I can test with the on board soundcard, to see if the issue persists withn non-USB microphones, but this might take a while (unless I can find that damned adapter for my headset to not use it's puck :)