One could modify this experiment to have very obvious effects. For example:

- Run the amplifier output through a banana or mud. Even if this somehow works and you can hear the sound, you’ll probably smell it as you cook and/or electrolyze your conductor :) (The banana likely works because the load impedance is very high in the experiment they did. The load impedance with an actual speaker is typically in the ballpark of 8 ohms. I admit I haven’t stuck a pair of multimeter probes in a banana lately, let alone done a proper I-V or AC impedance measurement.)

- Use really long cables. It’s not especially rare to be able to hear and even understand AM radio that gets accidentally picked up on a long cable and converted to baseband by some accidental nonlinearity in the amplifier.

- Use the actual outdoor mud on a rainy day as your conductor. I bet you can get some very loud mains hum like that.

Even audiophiles can probably identify these effects!