That's not normal, would appear stereo mix is fine, could I assume that regular playback is fine as well?
If that's the case it just narrows us down to Microphone and it's plugged right into the jack.
My questions at this point would be, are you boosting the microphone and what kind of microphone is this?
(I believe you might not be getting the right signal to noise ratio, under-powered microphone)
(or like said faulty chip, need more to go off!)