Awesome mod, nice one for who loves to build old airplanes with piston engines =D
But well. i've detected problems =/ , hope my review can help you;
KAX seems to work fine with the reverse system and sound, it uses the same plugin as you do (firespitter), so it may not be the problem.
Airplane Plus has issues with this, maybe it's how the code behaves, i've tryed working on this but since it's a "try and error" it was a no success =(
https://youtu.be/tyLVLm4tvFw
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Well, i've managed to change the audio of the engines with my ones, if you want them i'll need to fix some issues with the pitch of the audio first =P
I'm using ModuleManager 2.6.25