Comment 18 for bug 1365769

>>
>> >
>> > On a machine without fixed connecting audio devices like internal
>> > microphone or internal speaker, and when there is no external audio
>> > devices plugging in, the default source/sink is alsa_input/alsa_output
>> > and there is no input devices/output devices listed in the gnome
>> > sound-setting.

https://launchpadlibrarian.net/184055917/PulseList.txt

Seem line in is active port when all ports are not available
Pulseaudio won't switch away from line in but gnome won't show line in
since it is not plugged

ports:
analog-input-microphone-front: Front Microphone (priority 8500, latency
offset 0 usec, available: no)
properties:
device.icon_name = "audio-input-microphone"
analog-input-microphone-rear: Rear Microphone (priority 8200, latency
offset 0 usec, available: no)
properties:
device.icon_name = "audio-input-microphone"
analog-input-linein: Line In (priority 8100, latency offset 0 usec,
available: no)
properties:

active port: <analog-input-linein>

https://launchpadlibrarian.net/184417469/alsa2.txt