Followup --
Recall that this problem began with the update to Windows version 1709.. Earlier I had concluded that the SoundMax driver installed on my Lenovo desktop was incompatible with version 1709, so I wasted a lot of time trying the enable the generic driver, installing sound cards, etc. Now I have re-installed the SoundMax driver and the audio system works perfectly.
So it appears that at least the latest version of the SoundMax driver (for Windows 7) is NOT incompatible with Windows 10, version 1709.
Also, because of the vigorous troubleshooting in recent weeks, I wondered if I had inadvertently caused the change in the registry that proved to be the problem. Fortunately, I had made a backup of the registry early in this process. I just looked at that and see that the "LocalSystem" entry was already there. This suggests that it was inserted by the Windows update.
Just FYI.