This sound thing happens to me as well: I normally fly the Niki and when I take off the headset (oculus rift cv1) and put back on, the engine restarts, then the engine sound comes back on. At times, though, when I have taken off the headset multiple times during a mission, the engine sound does NOT restart and you hear everything else, EXCEPT the engine sound. It is like when someone shoots your engine and you have no sound. You clearly have a working engine, just no sound. I have not figured this one out!