Thanks NicoD. I watched your video review of the M4 yesterday; it was super helpful!
If mic recording works through pulseaudio, that would be fine; I'm just more familiar with plain ALSA.
Looking at the schematic, there are actually two microphone inputs connected to the codec chip. The 3.5mm plug is a TRRRS (five conductors) which has stereo out as well as mono mic in. I'm not sure if that is a standard "pinout", but even if it is, I would guess that a plug-in mic is unlikely to work unless it is a set of stereo earbuds with a mic in the cord, like a cellphone accessory. From the wiring it looks like it may be compatible with a TRRS (four conductor) plug as well.
There is another microphone input next to the 3.5mm jack, which is just a pair of holes at standard 0.1" spacing, so you have to solder a connector or wire the mic directly to that one.
I can see that all of these are connected to the codec in a reasonable way. I just worry that the supplied drivers may not have been written to configure the codec registers correctly for recording. It's the kind of thing which often gets missed because few people use it.
If you have the basic ALSA utilities on the board, could you dump the output of "arecord -l"? That would tell us if the software believes that it has a microphone input. If it doesn't list any capture devices, the mic definitely won't work; if it lists some, it might work