But plugging the analog hole on audio, stereo even, is impossible. The only reason HDCP was even a remotely plausible idea is that it's very difficult to captur

But plugging the analog hole on audio, stereo even, is impossible. The only reason HDCP was even a remotely plausible idea is that it's very difficult to capture 1080p 30fps from say the column drivers of a LCD. Capturing a 44kHz audio signal by snipping the lead off the loudspeaker and putting it into your recorder is very, very trivial. As far as RF engineers are concerned that's pretty much DC.