LOL! This happens to me quite often. I looked and looked and tried various things leading me to post to a forum for help. I then find the solution right after posting!
Here's the answer from here:
https://wiki.archlinux.org/title/Chrome_OS_devices#Fixing_audio
For my Chromebook C720P, I edited the file below:
/etc/modprobe.d/alsa.conf
to say this:
options snd_hda_intel model=,alc283-dac-wcaps
And then bingo, the internal mic works!