@DevonB Yes! That was it. Thank you! I had changed the interfaces count, but not added the extra lines.
I have moved on to a different ESP32-based setup now, but I just dug out the XIAO and tested my INMP441 mic on those pins with the InputSerialPlotter example and can confirm that it works!
The MEMS i2s mic pins are fixed and as follows:
SD / DOUT - 8
SCK / BCLK - 2
WS / LRCL - 3
LR / SEL - can either be LOW (ground) for Left or HIGH (3v3) for Right.
A heads-up for others: If you update your boards in the Arduino Boards Manager, that variant.h file can get reset to default and you’ll need to amend again. I feel like this should really be enabled by default.