Comment 43 for bug 1025388

For a fix that worked for me, see https://askubuntu.com/questions/1218136/internal-microphone-doesnt-work-when-using-headphones/1285982#1285982.

The fix:

 1. Run

  sudo apt install alsa-tools-gui

  hdajackretask

 2. Select the appropriate sound card up top in Select a codec.

 3. Then make the Black Mic (headphone jack) override Not connected.

 4. Lower right corner, select Install boot override.

 5. Then reboot and test.