1. enter in settings
2. in mumble configuration window make sure that in "Compression" part both Quality and Audio Per packet are not "RED" (move sliders and see when its get gray)
3. In my case, in both AUDIO INPUT and AUDIO OUTPUT (far left) the "Exclusive" BOX is not checked
4. When in game, do not close this windows on clicking Apply and Ok, just klick apply, do not close mumble and go back to game on ALT+TAB
That work for me