[korethr]

It still strikes me as plausible. Sub-harmonics are a thing; it is entirely possible to leak data about what the CPU is or isn't doing down into the ultrasonic or audible bands.

As an example, one of my computers leaks a ton of noise into the onboard audio that quickly becomes audible with a moderate amount of gain. So much so, that I've learned to recognize changes in the noise pattern from various activities (shuffling windows around the gui, launching a program, compiling a program, etc).

How practical a real-life, working use of the method described here will depend in no small part how much noise the device being attacked casts off. There's some pretty bad devices out there.