I recently went though two issues with a MK12D and were resolved by Narco. The first one was that I would also loose
sidetone and not be transmitting

We could never find the problem in the avionics shop on the bench

Wow! This would be a huge bummer, having already spent ~$400 to
replace a transmission switch diode??? I was, however, able to
duplicate the problem with a "known to be working" TKM radio, and an
old "original" Cessna radio.

However, I started having another problem with the receive audio, where it would just drop out. I got in the habit of pulling the volume knob out to be sure I still had audio. And several times I would not.

So many things to try and so many different scenarios for utilizing my
passenger with the handheld, that I don't recall playing with the
volume control (pulling it out)... Shoot!!! I'll have to try that one
next time I go up!

Thanks a lot for your response. The feedback from these groups is very
helpful and gives me things to try that I may not think of or forget
during this frustrating process!

Best Regards,
Todd