System sound output keeps changing.

I have 4 sound outputs available on my PC. See attachment. They all work ok but they keep changing. For example, the first that is selected as shown will play on my VX3276 primary monitor. After several minutes of PC use with no sound being used, I open a file that includes sound, and there is silence. I open the settings again, the first output is still selected but if I change to another, say "speakers", then the sound plays over my remote speakers. Once again, after a few minutes, even doing nothing on the PC, I open a sound file and the speakers are silent. The settings still show "speakers" but I click one of the other output options and the sound is heard. If I keep the audio file playing, it will continue playing forever until I close the file. Only after a short time of no audio will it have changed the ouput the next time audio is required.
Can anyone advise why the output keeps changing itself? Ideally I would like the audio to default to one output only every time it is required.

I posted this back in August but received no replies. The problem is still there so can anyone suggest anything? All drivers are up to date.