I have now tested the microphone in the 'tester' user account. The results are the same.
First, a snapshot of the PA recording tab when attempting to make the Skype test call:
Secondly, a snapshot of the devices listed when the test script is run (I selected the second device in the list):
Finally, a snapshot of the PA recording tab when using the test script:
Skype did not play back my message and when I played the .wav file in VLC, nothing was audible.
This probably has no bearing. After logging out of 'tester' and logging in to my normal account, the normal account did not load correctly and when I tried to leave, no 'leave' options were displayed. I had to restart using REISUB.
I have an older version of my normal user account now after reinstalling from a remaster. That has not improved the situation. One thing I have noticed is that recording and/or playing back with the headset on seems to increase the volume set for the internal system; the volume set for the headset is normally louder than the volume set for the internal system.