Thanks for following up @TE-KarlKomierowski ! I'll look into the PDM microphones.

Thanks for looking into it @TE-KarlKomierowski !

I'm using an Adafruit breakout of the popular Knowles SPH0645LM4H
https://www.adafruit.com/product/3421
https://learn.adafruit.com/adafruit-i2s-mems-microphone-breakout/pinouts

Generally I2S mics require two clocks, a word clock (or LRCK in this instance) and a bit clock. The bit clock is used to tell the mics how often to transmit, and the word clock (or LRCK) is used to tell which of the two mics to transmit.

The mainboard has an I2S0_LRCK pin (D25), but I'm not sure how to enable it
https://developer.sony.com/develop/spresense/docs/introduction_en.html#_main_board

I did some research, and it looks like the SPR_I2S0_LRCK pin is not pulled off of the extension board. Without this pin, Digital MEMS microphones can't function.

Located on page 3, extension board, pin 33
https://github.com/sonydevworld/spresense-hw-design-files/raw/master/CXD5602PWBEXT1/schematics/CXD5602PWBEXT1_schematics.pdf

Any advice?

I'm having the same challenge. I'm trying to use 2 digital MEMS microphones on D01 of the expansion board.

What LRCK pin should I use for this example?