Addendum and apology.
I had somehow missed Shankar C's earlier suggestion about simply hitting the HOME key after the volume mixer window has been opened using WinKey+R and typing in sndvol.
That method is simpler, better, and more foolproof as it unmutes and puts volume up to 100% in one key press. If you want to lower the volume from 100% you can then use Down Arrow until you reach the volume that's desired.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore