I have the same problem and its getting on my nerves! My microphone works normally, but in BF it doesent. I did setup the voip, and enabled it in the options/audio tab, the tarnsmit/receive setup is ok, the treshold is also fine. I get into a server, my mic works for about 30 seconds and then the little microphone icon to the right of my name disappears, like i didnt have any microphone. I disconnect the server, and see that in the options/audio tab the voip is now disabled, and when I enable it and run a mic test i cant hear anything at all.
The sticky thread didnt help also. I did all the things required in the dxdiag, made the voip server excluded from the firewall block list and the problem is still not solved. So any help would be appreciated.