Thank you for replying so quickly.
My TV is old, and the sound is poor. So I bought a cheap sound bar that doesn't support 5.1
I've noticed this volume going up and down problem is irritating when I watch a recording of PBS News. Perhaps it's them.
My set up is simple - optical cable from the box to the sound bar. The audio settings on the box are all "stereo".
Generally I find OPTIK extremely frustrating, I wish there was a "Press here and you'll go back to exactly where you left off."