If PulseAudio recognises the microphone is unplugged it will try to avoid using it for audio capture. The “unavailable” device can be manually set to be used , but some of the PulseAudio modules will try to switch audio to a device detected as available (plugged-in)
or a device without jack-detection and unknown availability like the Internal Microphone
Linux has a tool to retask audio connections and change jack-detection settings
You might need to install package alsa-tools if it is not already installed
Then run the retasking tool
sudo hdajackretask
I am on a desktop system so I can show how to disable my “Rear Mic” as you would want to disable your “Internal Mic”
And disable jack detection for “Front Mic” as you would adjust your external “Microphone”
Locate the Microphone pin to be disabled in hdajackretask
Click to enable the Override
Select Not Connected from the drop-down list
Then click on the Option Advanced override on the right side of window
Locate the External Mic jack connection
Click to enable the Override
And change setting for Jack Detection to Not Present
Click Install boot override so the override settings are applied when system is booted and reboot system to check if the retasks have worked
If the retasked connections are effective only the “Microphone” port should be visible with no indication of it being unplugged or unavailable
