I have the exact same problem, the omly differnce being that I built my PC.
But my mate built more or less exactly the same PC as me (we built them at
the same time in fact) and doesn't seem to have this problem. I've tired
everything, this doesn't seem to be a very common problem cos I've not found
it on any forums. But windows narrator is muffled and so is any other mic
feed sent over the internet.