Intermittent loss of engine sound

Hi

I wonder if anyone can help with this sporadic but irritating problem? Very occasionally my engine sounds disappear for absolutely no reason although all the other sounds remain - wind sounds, switches, gear and flap sounds etc. It sometimes happens when I change from cockpit to spot view or back, but there is no pattern to this problem and I can't seem to identify anything else that leads to it happening. It doesn't happen on all flights or with any specific aircraft, default or addon! Any ideas would be appreciated thanks.

Bill