Thanks for attending to this issue and the bug report link. I did post a message there, with a full description of the problem. However, I haven't had a response yet and continued to trouble-shoot on my side.
I learnt that the Steelseries Arctis 7 wireless headset (USB) that I'm using sometimes give problems on USB 3 ports. While I didn't have a spare USB 2 port, I plugged it into another USB 3 port on my PC, in fact a spare port on an Inateck expansion card, which I also use for an Oculus Rift.
This seemed to have solved my problem, as I get no more error messages in X-Plane 11 and sound always works, even after leaving the game and re-entering, which was what triggered the problem before. Perhaps you can add this to solutions for people having problems with the Steelseries Arctis 7 headsets.