- Run 'alsamixer'
- Press F5 to go to 'all'
- Change Channel to 4ch, make sure your respective volumes are > 0.
This fixed it for me in my situation. Hope this helps!
Edit: As a followup, this appears to sporadically work, but not all the time. The mic seems to work for about 5 minutes after this, then no longer works, and I have to go back in and flick between 2 and 4 channel mode to seemingly 'activate' it again.
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