I have kept an eye out for the issue frequently now, and I believe one trigger is the hard power cycle of the sound bar.
For automatically changing Windows audio settings I think I have found a solution with Sound volume commmand line tool for Windows (see: windows 10 - Switching default audio device with a batch file - Super User)
However I have not identified the method to be able to programmatically change / set the audio options in Plex HTPC. If the location where these settings are stored was identified I could probably use task scheduler + batch file to trigger setting the right values on system boot and system resume from sleep.
