Increase gain of electret microphone amplifier powered by aircraft radio?
Here is something to try:



This circuit will load Vout with a current proportional to the microphone sound.  It's not totally clear from your description that is what the input to the existing system wants, so this might not work.  This is assuming the existing system powers Vout, and reacts to the current signal drawn from it.

R5 and C1 filter the AC from the input voltage to make a reasonably steady average DC voltage.  This voltage is then used to run the electret mic, with R6 being the load.  The mic signal is then AC coupled thru C2 into the amplifier section.

The amplifier is similar to what you already have, but with a more predictable gain and operating point.  The emitter of Q2 should be held at around 4 V when there is no sound.  Vout minus this 4 V causes a current thru R1, which is the output signal.  That current will be perturbed by the signal from the microphone.

R3 and R4 bias the amplifier stage by providing DC feedback.  This DC feedback is high enough impedance that it can be perturbed by the microcphone signal.  

Its not clear how much current change the existing system is expecting for a normal amplitude.  You'll just have to experiment.  Hopefully this circuit provides either enough or too much gain.  To reduce the gain, increase R2.

R1 should be a "&frac12; W" resistor if you keep the current value.  Again, it's not clear what current range the existing system is expecting.