Is EMI noise in electrical system inaudible?

There are many ways high frequency interference can become audible. One way is simply modulation. Two inaudible. signals mixed on non-linear element, like semiconductor junction, will produce new frequencies, that might be audible. The phenomena called "rectification" is less known. Most of op-amps and perhaps many amps suffer from it. Amplifier, being frequency limited cannot amplify very high frequency. They filter it (taking average) resulting in zero output. That is as long as amp’s negative and positive slew rates are identical (almost never are). As a result of it there will be some DC value on the output, that will be proportional to offending interference amplitude. (AM or even FM signals). That way instrumentation amplifier designed by my coworker received local AM station, being bandwidth limited to only 1kHz. We can stop it by using passive filtering at the inputs, but nothing is ever 100%. Wire that is shorter than 1/10 of the wavelength of offending RFI is a very poor antenna. Does it mean. that is not receiving any RFI? Of course not - it will still receive tiny amount. We can reduce it to inaudible levels by using twisted pairs (even exposure to electric and magnetic fields, resulting in cancelation), shielding, filtering, keeping connections short etc.

So what the heck is a harmonic and how does this interfere with good sound?

@jumia . If you mean harmonic distortion it is a deviation from original shape of the signal.

Oh my god really ?