SOLVED: Windows 10 mic input not working

I hope that this post rises in the search-engine results because I spent days trying to fix this issue, then found the solution through careful experimentation.

MY PROBLEM

My family and I are stuck sheltering in-place because of coronavirus and my kids are all attending online classes. So I had to dig-out several old HP desktop computers, and the first thing I did was refresh them with the latest Windows 10 '1909' operating-system. Everything seemed to work great.

Then we started to have issues where everyone was trying to participate in video-conferences at the same time (and in the same room) so I bought everyone corded headsets. But the microphones wouldn't work! I swapped headsets around, even tried my known-good headset from my work, so I knew the issue wasn't the headsets -- it was the computers.

NOTE: these computers used the old-style pair of Headphone vs Mic inputs, i.e., it did not use the newer combined input (like for smartphones). I'm not sure if this made a different to my eventual solution but it did help me to troubleshoot.

HOW I TROUBLESHOT

First I opened the Device Manager and saw that Win10 had installed (2) sound/video drivers:

• High Definition Audio Device
• IDT HD Audio

The IDT driver was dated 2015 so I tried the 'Check For Updates' function, but it reported that it was already the latest version.

Next,  I opened the Win10 'Sound' settings. I noticed that when I plugged/unplugged either the headset or the mic plug, the list of available Output or Input devices would auto-update. So this told me that the IDT driver was correctly detecting the presence of the plug; this was a good sign!

MY SOLUTION / EXPLANATION

I eventually realized that the new Input (that appeared in response to plugging-in the mic) was labeled "Line In" rather than "Mic In". If the driver thought I had attached a line-level device, e.g. an external CD player, then it wouldn't apply the necessary amplification for a very low-level device like a microphone. Something was interfering with the IDT's ability to fully detect the status of the mic input-plug...

So I tried disabling first the MS-provided "High Definition Audio Device" device. Everything still worked but the mic still failed to be detected correctly. At the very least, this demonstrated that the 2 drivers were redundant. Next I tried disabling the IDT and re-enabling the MS; now the mic plugin wasn't detected.

Finally, I disabled both audio devices -- then re-enabling only the IDT driver. It worked! In the Sound settings the new Input was labeled "Mic In" and the headset mic now worked. So, apparently, when the IDT driver starts-up, it is blocked somehow by the presence of the default MS audio-driver.