Stereo mix has become "unavailable" again.

I must assume that an update is responsible.

I have no idea how to reinstate it, and would appreciate any help to do so.

The Dell Optiplex 780 (what you show as My Computer) is not supported past Windows 7 by Dell. This doesn't mean it can't or won't run Win 10, just no Dell specific drivers available and the availability of non-Dell issued drivers is "iffy".

Thus if it did work and now doesn't, it sounds like whatever update you got removed or disabled Realtek. Some newer versions of Windows do not support older hardware such as this and if it was upgraded to a later than 1803 Win 10 that could be the reason, and usually no options other than reverting to the previous Win 10 version.

If its not a new Win 10 version issue, check the Device Manager and see if its still listed. If it is, "refresh" Realtek by uninstalling it in the Device Manager, but DO NOT uninstall the driver. Then restart Windows and see if its detected.

Thank you fireberd

I had a poke around and it's now ok.

Sorry, don't remember exactly what I did. My short term memory is not very clever, and I suspect it will get worse.

I've had my three score years and ten and coming to the end of bonus extra decade.

Never mind, it's not forever, it's only for life!

It's in Settings > System > Sound > Manage sound devices > Device properties > Input devices > Stereo Mix > Enable/Disable.

That's the first thing I looked at.

It was "greyed out" so I didn't have the option to do so.