alt Hacker NewsFor USB input and SPDIF output, all that you need is a TOSLINK TX module(s) or a couple of capacitors and a resistor if you want coaxial SPDIF. For I2S output, the PCM5102A modules that you find on Amazon work very well, with very reasonable performance (SNR >100dB, THD+N ~95dB).
For 2.1 configurations in a pinch, the firmware includes a software DAC that's more than adequate to drive a subwoofer, so only one external DAC is needed.
Maybe start by omitting the part in the first paragraph about how it acts as a USB sound card. :)
I mean, I know what you meant, but that's pretty misleading phrasing for many people.
It's not much of a stretch to think that most people interpret a "USB sound card" as a thing with analog audio on one side and USB on the other side. But other than the subwoofer output, we don't have any analog IO on a Pico running this firmware.