IQAudio's Codec Zero on Raspberry Pi 0
Codec Zero (formerly from IQAudio) is a RaspberryPi hat with various audio I/Os, including microphone. I got some stuff out of it but ultimately ditched it when I couldn't get a good enough quality.
#Issues
I liked this board at first: it's easy to plug in (it's a hat), it's small, there's a dtoverlay available. I actually can't recommend it however.
#MEMS microphone is not sensitive enough
I had to tweak several levels in the audio chip configuration and add +30dB at playback. Signal volume was limited but OK-ish; noise volume was similar however. Since this was white noise, filtering didn't help much or started damaging the signal.
After my attempts, I picked the first I2S MEMS microphone I had ready (i.e. with pin headers soldered), a GHY-SPH0645 breakout board. Found the wiring rule and went for the dtoverlay that adafruit advised. Without any tweak, the SNR was at least 30dB better.
#Unusable microphone on JACK input
Since I couldn't get something usable from the MEMS microphone, I tried using the JACK input. The MEMS microphone is immediately muted as expected but after that, I never got anything from the JACK input.
I tried with several 3- and 4- pins microphones which work elsewhere. I only had the same noise.
#References
https://github.com/iqaudio/Pi-Codec
https://datasheets.raspberrypi.com/iqaudio/iqaudio-product-brief.pdf