When talking to people over Mumble/Teamspeak/Ventrilo, I have a few who are broadcasting all of their audio output (e.g. music playing in the background, game sounds, etc.) whenever they are sending -- in addition to the microphone signal.
I don't have this problem myself, so it's hard for me to troubleshoot remotely, but I'm interested in collecting solutions for this problem for Windows XP, Vista and 7, so that I can link them to this question.
Links to other related SU questions are highly welcome as well, but I couldn't find any. Most people seem to try to either get microphone to feed back into the audio output or turn exactly that off, which is different from this problem. I'm talking about the opposite problem, the output being fed into the input.
I'll assume that it has been ruled out at that point that it's just the microphone picking up the actual audio waves from the speakers. The person is wearing headphones and probably using push-to-talk.