Internal microphone not showing up on Lenovo Yoga S740-14IIL (18.04)

I followed the advice options here: Internal microphone not working

tail /etc/modprobe.d/alsa-base.conf

options snd-cmipci mpu_port=0x330 fm_port=0x388
# Keep snd-pcsp from being loaded as first soundcard
options snd-pcsp index=-2
# Keep snd-usb-audio from beeing loaded as first soundcard
options snd-usb-audio index=-2
options snd-hda-intel dmic_detect=0

alias snd-card-0 snd-hda-intel
options snd-hda-intel model=auto

but I'm not even seeing a microphone in my settings:

I have the same problem with the same laptop, running Debian and kernel 5.7 and below.

I have not managed to fix the problem, but as a workaround, if I plug in specific headphones (ones with a four-connector TRRS plug), the built-in microphone is mysteriously detected and can be selected as an input device.

Edit: With kernel 5.8 and Sound Open Framework 1.5.1 (https://github.com/thesofproject/sof-bin) the built-in mic is picked up without any messing around plugging things in.