I'm not sure why a TV tuner card would be doing anything with PC generated sounds (assuming it can), but that could be part of the problem.
I think the English word you are looking for is mute. If there are any entries in the volume control for something like 'auxiliary output', you might want to try muting it. You may need to go into the options to enable some entries for playback / recording in the sound control applet (if you enable everything, and then start checking off the mute boxes, you may be able to determine what is outputting system sounds to your friends).
In the multi-player game and voice chat programs you have, are there any options to select the source of your voice? Perhaps they are set to 'what you hear' rather than 'microphone', or something.
For your keyboard/speaker test, were you changing the volume on the speakers directly, or through the Windows volume control?
Since your friend did not hear a difference, I assume you you were turning a nob on the speaker itself, which means the sounds are coming from your system, not some phantom microphone somewhere. That implies that if you press a key softly, you should hear a difference yourself as you change the speaker volume (either through the windows master volume control or at the speakers).
