Mumble has automatic noise suppression which eliminates repetitive/constant background noise (aka fans and stuff) so that's why the mic sounds better in mumble. With that in mind, a significant part of that background noise could very well be your motherboard's built in audio and not the mic itself. Watch the last part of this video, starting at 2:15, to hear the difference between motherboard audio and a cheap (think $5 dollars) USB soundcard on a Sennheiser PC 230 headset.
https://www.youtube.com/watch?v=QnnOALkQqLA#t=134
EDIT: I didn't realize onboard audio came with built in noise suppression now, so if that's the case then listen to TechDude instead of spending money. :o