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.
I too have a cochlear implant with the 2.5mm jack adapter, and I actually prefer headphones to the jack adapter, mostly because the color of the sound seems to be a lot better with headphones than the 2.5mm jack - the direct audio signal seems harsh and not warm. I listen to a lot of classical rock, and this may be part of the reason, it just seems to translate better over headphones than direct input.
