A DC-DC converter always uses a variable duty cycle to maintain the target output voltage (or for CC, current). Without it, the voltage would vary wildly depending on load.
For something like an audio amplifier, obtaining precise power supply load would in turn give you a curve over amplifier load, which effectively gives you the speaker amplitude. Input caps and filtering will likely remove the high frequency components entirely, but you might be able to construct at least part of the played waveform.
All good points. I would say that it's a fairly outlandish scenario where you are (i) close enough to the device to listen to the caps whining but (ii) can't measure actual voltages within the circuit (which could be a lot more informative) and (iii) can't just listen to the audio output of the device directly.
Acoustic noise is one thing, but it's not at all outlandish to be within range of the EMI emitted from the same power supply which tells the same tale. What is outlandish is thinking anyone bothers listening in. :)
For something like an audio amplifier, obtaining precise power supply load would in turn give you a curve over amplifier load, which effectively gives you the speaker amplitude. Input caps and filtering will likely remove the high frequency components entirely, but you might be able to construct at least part of the played waveform.