Sound input keeps changing

I am answering myself, having cured the problem.

Safe Mode did not help. But at some time in the distant past, I had installed (1) Soundflower and (2) Corel Painter, but found that I did not use or need them. Soundflower turns out to be a kernel extension; Corel Painter similarly inserts a number of sound and language extensions (Why? Strange for a painting app!). Searching on the Net, I found Terminal scripts for removing each of these; (I suspect that the best script to use in each case. depends on one's particular MacOS.)

I have had no more trouble with Sound Input changing. These kernel extensions (and no doubt there are others that have similar effects) can obviously cause hard-to-explain troubles.

Thank you, AlwaysWantingToHelp. The trouble is that actually the phenomenon is pretty random in occurrence, not as consistent as my previous post suggested. So I would have to keep testing in Safe Mode for a lengthy period of time. But I shall try it as soon as I have a slack period, and shall then pick up this thread again.

Solved!

Two or three years ago, for some reason that I can no longer recall, I installed Soundflower. I have now uninstalled it, and so far my sound input setting has remained where I set it. Long may it stay so — bearing in mind the previously rather random appearances of the fault.

Uninstalling Soundflower is not that straightforward. In my system (Mojave, latest minor update — 10.14.6) the following procedure, found on another website and slightly modified, has worked. well:

Uninstall Soundflower

From the Finder, HIT Shift-Cmd-G and TYPE: /Library/Extensions

Then inside that folder, look for a "Soundflower.kext" or ‘SoundflowerSigned.ktext” file. If there is one, DRAG it into the trash (you may be asked for the admin password).

REPEAT, typing in this folder path: /System/Library/Extensions

Then OPEN THE TERMINAL APP

Type this line, entering your password when asked. sudo touch /Library/Extensions/

RESTART your computer.