It is completely unclear how HF harmonics from a power cable get into the signal circuit and how 60 Hz buzzing can affect the music signal except to cause something like the same buzzing?
This is not an advocacy for expensive power cables, but there are many ways the harmonics in the AC to get into the signal. Whether they do or not is a different question. The most obvious one is via the power supply, especially in a low feedback amplifier. Primary power supply harmonic is 120Hz, but with all the linear supplies, there are harmonics at many multiples of that frequency, certainly up to several KHz. Those big transformers audiophiles love get rid much of the really high frequencies.
Those high current peaks from the power amplifiers generate harmonic noise on the AC line that can get into other power supplies.
Those high current peaks can generate higher frequency EMI that can get into signal lines (at least a justification for shielding).
I don't see a lot of justification for the cost or claims about most high end power cords. Most of these power cord / cable designers have little knowledge of electronics which is evident in their claims. It works because their customers do not either. Vicious circle.
That 60Hz buzzing is not always just 60Hz. It is only something that happens 60 times a second. There can be rich harmonic content in that 60Hz buzz. Now that take 60Hz and harmonics and modulate a music signal with it. Now you have stuff all over the place.

