First off, this isn't the usual "physical" feedback of a speaker being too close to the mic. This mic is part of a headset so there's no way for the output and input to overlap and cause feedback. This mic has worked perfectly for me in the past, but I recently re-installed my OS and it hasn't worked since.
It seems as if my audio out is getting redirected to microphone in. If I open up and sound recording program while I have some audio being output, the output will get echoed back in through the microphone channel, although any actual microphone input is never picked up. I can blow or scream into the mic and there's no indication at all that Linux is picking it up.
I'm running ArchLinux with ALSA. I've gone into alsa mixer and played with just about every channel in every way I can think of, and none of the options seem to fix the problem.
How should I fix this? I use my mic pretty much constantly when I'm on the computer, being without it sucks.