There have been lots of problems with the A6 and I believe the A24 to be similar. ICOM aren't interested, so you have a chocolate teapot as far as a backup radio.
One thing to check: Are you using the PTT on the side of the radio or a seperate one connected to it.
I believe the one on the side turns on the internal mic and you need to use an external PTT to get your headset mic to work.
see here for more details
http://www.pprune.org/private-flying...-headsets.html