Interesting! Do sounds and voices played through that headphone jack sound higher quality than sounds picked up by the local microphone and relayed to you? What's the frequency range that you can pick up with it? Can you hear sounds >20kHz or <20Hz?
Also, 2.5mm (like old headsets) or 3.5mm (like standard headphones)?
When I said "local microphone", I just meant the microphone that picks up local sounds, as opposed to anything you might plug into the headphone jack.
Thanks for the explanation; very interesting.
Have you considered testing the frequency response of both options? See http://www.engr.uky.edu/~donohue/audio/fsear.html for test sound files. You could play those files via speakers and via the headphone jack, and see if you get drastically different results for certain frequencies.
Also, 2.5mm (like old headsets) or 3.5mm (like standard headphones)?