Win 10: sihost.exe "Unknown Hard Error" glitch

After just rebooting, to try and get rid of this out of nowhere "null icons" thing...well they're gone, but now I get the infamous pop-up "sihost.exe System Warning" which says "Unknown Hard Error." If I click "OK," my taskbar goes away and I'm dead in the water. If I don't, the PC seems OK except for the tiny detail of no sound now. And media player says it can't find any of the songs I have (that led me to realize I have no sound), even when I know it's there. The only somewhat suspicious thing I see running is this "Musnotifyicon.exe" now. I hit End Process but doesn't seem to matter.

I checked and Win 10 has not installed any updates recently.

I have not installed or run anything different lately.

I ran Malwarebtyes, found nothing. Some 'net searching suggested the following:

1 - Run sfc /scannow (as admin). Did it, it said it did some repairs (my hopes go up!), rebooted. No diff.
2 - Run chckdsk. Tried (as admin). Won't let me, says "Chkdsk cannot run because the volume is in use by another process. Would you like to schedule this volume to be checked the next time the system restarts?" I say yes and reboot. Doesn't run as far as I can tell, but not sure.
3 - Disable all non-MS startup items on boot. Except I "Stopped" all such services before this ever happened, so don't see how that could be it.

Any PC gurus have a clue?