[Stwerp]

Maybe you can describe this a bit more. The Great Seal Bug was used for wireless audio recording in the 40's, but this operated by using a microphone/cavity to modulate the load of an antenna; thereby embedding audio in the reflected fields.

Bell did a similar demonstration in the 1890s using a mirror to embed reflections in ambient light and demonstrated wireless audio transmission over 200 meters.

However, I'm not sure why audio would cause a frequency shift in a radio circuit. Perhaps you are meaning the antenna will be perturbed and that could possibly be recovered? I'd be interested to know.

[jacquesm]

In the times that I was still building radio transmitters (for a very illicit living, selling them to pirate radio stations in Amsterdam) I had to hot-melt each and every long wire and coil in place so that it wouldn't vibrate.

The oscillator circuitry of a transmitter is (even when crystal controlled) sensitive to mechanical perturbation, which typically leads to spurious AM and FM modulation of the outgoing signal. To demonstrate the effect I once held a half our session on air with a guy on the other side of the city by just talking to the circuit board.

In a PLL or crystal controlled transmitter modulating the carrier in such a coarse way is much harder. Typically the modulation is done using a capacitive diode (a varicap) which is a diode whose capacitance changes with the reverse voltage. Because this voltage has to be applied to the diode somehow (in the days before SMD) this meant that that wire was again susceptible to microphony because air pressure on the wire changed it's location relative to the ground plane and that caused a measurable frequency shift. Not nearly as big a shift as in the older stuff but it was definitely a factor.

Wifi radios as much more robust than the stuff that I built. But I suspect that given a sensitive enough detector a residual audio component might be extracted from an otherwise non-audio signal by direct interaction between the sound waves and the transmitter hardware.

In a nutshell, it is very much harder to make something that does not exhibit microphony than to make something that does. You'd have to take that into account from the beginning of the design.