They used an SMD Mic with internal amp/chargepump. The formfactor is 3.76x2.95. I have a link to a mic I’ve already used succesfully in the bottom module.
You can of course buy it from other shops and use other mics, but I’ve tested this one
https://lcsc.com/product-detail/Electret-Condenser-Microphone_Partron_AM4311T42A0_Partron-AM4311T42A0_C267056.html
But it isn’t easy to solder, because the pads on the board are very fragile