However, most audio output (at least back in the nineties and naughties) is not capable or creating sounds reliably above 16khz. They almost always have a low pass filter on which cuts it out. Apparently most dell laptop speakers cant even manage above 10khz

"You can't hear" doesn't have to be high frequencies though.

It can be more subtle like phase shifts and other "in audible range" fingerprinting.

While I've not used it for getting on 20 years now, I did do a lot of software audio engineering back in the day, everything from RF to DSP.