[gruez]

There's no "end-to-end drm" for audio. Any audio can be captured near losslessly by plugging in a USB-C to 3.5mm adapter, and a sound card on the other end. There's also no HDCP-like mechanism for sound, so if you're willing to put the work into it, you could make a fake USB DAC that produces a bit perfect capture of the audio output.

[out_of_protocol]

Just use DAC with S/PDIF output, and another one with S/PDIF input. Well, either way it's hard enough to block casual users, so... it works