Mac OS (High Sierra 10.13.6) volume control spontaneously runs; can I disable it?
Doesn't appear to be a hardware problem after all. Resetting the NVRAM/PRAM seems to have done the trick. Before doing this, the problem was occurring about every two minutes. I've now run the system for 24 hours without an incident.

Edit: the problem is back after three days. I would appreciate any ideas.

Edit: the problem turns out to be traceable to a particular set of earphones. Didn't happen unless they were plugged in. Changed earphones, no recurrence of problem. So, it was indeed a hardware problem, just not with the computer itself.