Hi and thanks for the replies!
I have found at least one problem, which might have contributeted, but I couldnt check it yet.
I have a mainboard with a nForce-Chipset. And the Nvidia-drivers I installed caused a whole lot of stange networking troubles.
E.G. I could not view any microsoft-page in the browser, or certiain other websites but the majority (like hitech's) worked.
Somehow the faulty drivers caused this, and now I'm trying to get it up and running again without those drivers. I could imagine, that the tansmission of those sounds from the microphone might just have been blocked somehow by these drivers.
I didnt get to check if this problem is gone now, and I wont be able to check this evening. But hopefully tomorrow.
Thanks again, I'll try the tune-to-yourself-trick to see if it works