Personal Audio Cables with phones may not work like you would like since there is not a microphone in the cable. I used the Hatis device until it quit working a couple months ago.
As far as the microphone ratio to the T-coil, I don't believe that is adjustable outside of the program. If you have manual controls for sensitivity and volume and use the PA cable, you can depending on settings, turn down the sensitivity to 0 and that may turn off the mic.
I have had pretty good luck with the T-coil setting. Hopefully the Audi can get your programs set better to help you more.
Steve
