Adding microphone to Unicorns?

Try single row sockets (see this post for an image: Raspberry Pi Pico - why does free space show as 848 kB? (and advice on how to add an SD card to an enviro indoor) - #11 by bablokb
Be careful not to solder for too long.

What are your requirements regarding the mic? There are cheap PDM mics around that just need two pins (no adc), but if you really want to record something for playback, you will need to do additional research. I use them for noise-level detection and for this purpose they are fine.