Internal Microphone not working and not mute

[Dave6822]

 The internal microphone on my Elitebook x360 doesn't work, it won't record.  I’ve had it a couple of months but only got the chance to start using it in the last two weeks.  It seems that there have been others with similar issues, so instead of start with the basics, here's what I’ve tried so far.
 
In the 'Sound > Recording' tab (right click speaker in task bar) the internal microphone (Conextant ISST) is enabled and default (always was).  The green bars do not light up if talking into the microphone, but one or two bars will appear when I tap on the display.
 
Internal microphone properties > Levels: microphone array 100%, microphone boost +36dB (max). I have tried other levels also.  The microphone does not mute by itself, as I read that others have experienced.
 
Internal microphone properties > Advanced: have tried using lower default formats, exclusive mode: on and off, signal enhancements: on and off.
 
In device manager, audio inputs and outputs: I have updated and then uninstalled and reinstalled the Internal microphone (Conextant ISST Audio) driver.
 
In device manager, sound, video and game controllers: I have uninstalled, downloaded and installed new drivers for Conextant ISST Audio and Intel(R) Display Audio.
 
The mic mute button on the keyboard (F8) is not on.
 
In command prompt, as administrator, I have killed the svchost task in the name of the computer and then run Malwarebytes - did not fix anything (this worked for someone else).
 
I have updated the BIOS.  I checked the BIOS to confirm the microphone was enabled.
 
None of the above has managed to achieve anything.
 
Other troubleshooting:
 
Windows 'Voice Recorder' will not record my voice, but it will record a faint tap sound when I tap the display (noting that gain / boost is on +36dB, so this should probably be deafening).
 
Another thing that appears odd is the Sound Mixer.  A third sound level setting appears with 'Name Not Available' (image below).  I don't know if this is a related issue.

 

 If I plug in a set of external earphones and microphone, the 'Voice Recorder' records and plays back perfectly.

It seems like the internal microphone is close to dead, since an external one works and all drivers are installed and working. Are there any other suggestions for what I might try?

Thanks in advance.