Problem with Windows 10 ver. 21H1 -- all audio input/output devices missing in Sound settings, red X on volume icon

For 2-3 days, I am getting no sound at all from my several audio devices/Windows computer here - I have TWO monitors with HDMI audio, plus headphones jack connected to motherboard audio card. I was able to fix it one time by doing lots of tinkering in Device Manager, but I am unable to replicate the fix a 2nd time.

So, asking here.

System is a desktop PC, running an Intel Core i3 Haswell CPU (2 cores, 4 threads), 16 GB of DDR3 RAM, on a ASUS H81M2 motherboard, GPU is AMD Radeon R9 390 (8 GB). Audio card is the built-in Realtek. 1 Crucial SSD, plus 2 HDDs -- WD and Seagate. Logitech Full HD webcam , Logitech wireless mouse, wired Razer keyboard.

The volume icon in lower right (Notification area) shows with a red X. In Settings of Windows 10 OS, Sounds, NO input or output devices show at all.

No sounds possible in any software, i.e. browser, actual OS does not make the Start Windows sound like it used to/etc.

Also, no recording or voice chat possible with my Logitech webcam (which has a stereo microphone), so no Skype, Viber/etc voice calls.

In Device Manager, nothing shows wrong - all audio, system devices appear to be working fine. In AMD Radeon GPU settings, nothing wrong shows either.

I tried the Windows 10 Sound Troubleshooter, but it unfortunately says it cannot identify the issue.

What can I do to fix this?

(Please do not suggest a reinstall Windows OS or Reset OS, I cannot do those.)