There is precedent for these types of acoustic attacks. One interesting paper from 2014 that comes to mind is this: https://www.tau.ac.il/~tromer/acoustic/

Basically, the idea is that even though clocks run at GHz (which in acoustics, would be impractical ultrasound), modular arithmetic (exponentiation) takes ~milliseconds to run, which means they produce acoustic signatures in the kHz range, which travels easily (and omnidirectionally from the kind of small aperture that the motherboard represents) in air and can be acquired with cheap microphones.