I have noticed, that you often can't hear the engine sounds of jets, even if you should be able to hear them.
I think i figuered it might have something to do with them going supersonic, as it seemed to work right when i limited the speed in the waypoints when testing.
Edit: nevermind that. it was just more subtil.
So i placed 4 jets passing me at 2km, 1.5km 1km and 0.5km distance at max speed.
See and hear for yourself.
especially good to see and hear is the part with the sonic boom. as soon as you hear the boom you should be able to hear the jet engine, but instead you hear it first 14sec later.
Tested the same with limited speed:
Audio poped in before the jets passed me, but still popped in. Not as bad as in the first example, but still not perfect.