I've been having microphone problems under Boot Camp 5 in Windows 7 Ultimate on my rMBP mid 2012 since the very beginning. I'm using mic driver from the latest Boot Camp support software 5.0.5033 and the device is Cirrus Logic CS4206B (AB 40). As long as I use Skype or any voice chat in Mac OS, everything is fine, but in Windows 7 there is a lot of background noise (mostly fan noise) in any voice chat e.g. Skype, Teamspeak, Ventrio or any other.
So under OS X, background noise (incl. fan noise) is filtered or suppressed and under Windows when the fan is on it's really noisy. I've also tried many different audio drivers (incl. those from the official cirrus logic website) and it didn't help.
MacBook Pro with Retina display, OS X Mavericks (10.9)