Humble apologies to all on this thread, and thanks for your trouble. As one of my old teachers told me a long time ago in his Glasgow accent, “Always check the obvious, lad. It’ll save you a world of trouble.”
He was right. I have been driving myself crazy, when the answer to the problem was very simple. My nice new 4K monitor has no speakers.
I’m going out to buy some speakers. Thanks again, all.