How to interface to a PDM audio source?
To avoid bit-banging, I think you can use SPI input to capture PDM data, but that would only work for one channel (mono input). This Texas Instruments app note describes something along these lines.

After some brief search I've found these off-the-shelf products which support PDM:

Codecs: Cirrus CS53L30, Maxim MAX9888, TI's programmable TLV320AIC3253

Micros with integrated PDM inputs: Silabs Giant Gecko range, Maxim  MAX32666

Analog Devcies PDM to I2C converter

That's just a few examples, there are plenty other products I'm sure.