[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.