Changing default subdevice configuration
Some sound devices have multiple capture subdevices (eg. hda-intel),
but pulseaudio only seems to detect the parent device, and only listens to subdevice #0.
ALSA may not be configured to use your preferred device as subdevice #0 by default.
Open the pulseaudio record meter (pavumeter --record)
Talk into your chosen device while you carry out the process and watch the meter.
Some channels are quite noisy even with no input (especially on integrated chipsets),
so we are watching for movements that correspond to our voice.
Use alsamixer -c n (where n is your sound card number, probably 0)
Press F4 to get to the capture console
Make sure the first capture channel is enabled ("CAPTUR" in red, press space to toggle),
and the volume turned up.
Check the first input source is switched to your chosen plug (Mic, in my case)
At this point, you should see the record meter bouncing in response to your voice.
I found that I had to repeat this procedure each time I wanted to use the Mic,
even though the settings "stuck" in the mixer, something in the soundcard wasn't initialized
until it was repeated. This also works if you use the "Options" tab in the standard mixer app to do the same thing.