It doesn't mandate any of that, it's an optional feature.

The data pins are repurposed for analog audio, so it won't work with hubs. You'd of course need a DAC for output and an ADC for mic input, but the point is to replace a headset jack, so you'd have those already.

https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%... (PDF, page 309)