MacBook Pro - no sound

Hi. Starting last night, I've had no audio from iTunes, YouTube video, etc. Occasionally, I get a short burst of audio, but never longer than about 5 seconds. Watching a video today which gave such short durations, I found that if I paused it, and then started it again, I again got another burst (sometimes). Strangely, the volume control icon at the top of the screen keeps resetting its output to zero, even if I've just reset it to full. Also, on the Sound page of System Preferences, under the Output section there is no listing for Internal Speakers - Just Boom, the Digital Out, and Soundflower (2 ch.), the latter of which I cannot delete from the Extensions folder. I do hear the start-up chime as normal.

I upgraded to Catalina, but again, no luck. I use BOOM to enhance the audio, and it seems OK, with it's output set to Internal Output. I called Apple, who got me to reset the PRAM and the SMC, but without any change.

Any ideas as what else to try? Thanks in advance.