Impedance matching headphones and carbon microphone adapter
Giving the headphone amplifier a 100 Ω load rather than 150 Ω is unlikely to cause any trouble. If you're really concerned then connect them in series for 200 Ω but make sure that they're in phase so you don't get the weird "hole-in-the-middle" effect when out of phase.



Figure 1. Image from Electret microphone amplifier for general aviation.

What's not shown in the schematic is the source resistance in the AUDIO+ input. If you want to generate more volume then you need to generate more voltage and current in that resistor. To do that you would need to decrease the value of the 100 R.

Try experimenting.

Note that most aviation headsets use a pair of microphones in anti-phase - one facing the mouth and one facing away. This helps to cancel out cabin noise without cancelling out speech.