I suspect the aircraft you're having problems with use military-style avionics, which have a different impedance than standard civilian avionics, both for the mike and for the speakers. The speakers should work a little, but you'll have to turn the volume full up just to barely hear anything. I've used standard military headsets in civilian aircraft, with a mike amplifier and with a civilian mike, and I had to turn all the volumes full up, both on the radios and the mixer box, just to hear. The impedance mismatch is too great to be easily overcome. DC does sell a dual impedance headset, which lets you change between the systems by just flipping a switch on the headset.