Here's a workaround, it's ridiculous and probably a bug (any Apple SEs in the audience please move along, nothing to see here) but it works: - Open the "Audio

Here's a workaround, it's ridiculous and probably a bug (any Apple SEs in the audience please move along, nothing to see here) but it works: - Open the "Audio MIDI Setup" app that comes pre-installed with the OS - Bottom left plus icon > "Create Aggregate Device" and optionally name it - Add your built-in microphone to this new device by dragging it from the sidebar to the left - Open your sound settings, and select the aggregate device as your main sound input After this it should no longer change the main input device when you connect something else.