I'm using bananian on the banana pi and I tried following your procedure, but without success. alsamixer is a part of package alsa-utils which I installed, and when I invoke alsamixer, there are no obvious indications of a microphone. I modified the /etc/asound.conf per another article on this forum (not sure that was the right configuration for an regular banana pi). After that alsamixer appears to have some settings that look like the microphone settings, but still not obvious which is the on-board mic.
Anyhow, when trying to record, using this, I get an error:
sox -t alsa default test.wav
sox FAIL formats: can't open input `default': snd_pcm_open error: No such file or directory
Anything I missed?