I use the Chalenge Custom sound pack. I've noticed that if I remove the VR headset and then put it back on, it will cut out the engine sound. Guns still fire, other sounds are there. But it's eerily quiet from the engine bay ;-) Stopping/starting engine in-flight doesn't fix. The only fix is to end sortie (or typically die, more usually), and then re-up a new sortie.
For those not using custom sounds...I don't think this has been an issue.