I recorded some stuff last night and there was no problem, but using my pc today, in the same setup, I get feedback. With no mic plugged in (but my mic mixer on), I get a high freq buzz, and even when I record with the buzz, the actual audio is weird sounding. Any ideas? I've had a problem like this before, it just kinda came and went. I can even mute all audio and leave the mic input on, so it doesnt even seem to be feedback, just some kind of interference or something.

Do you need the mic input on?

You could be getting a faint electrical charge in the air--turn off your 'mic mixer' if you're not using it.