"100MHz... we can't hear 100 KHz, how are we gonna hear 100MHz"
We won't hear 100 MHz, but RF can enter audio circuits via cables and get demodulated. It can result in offset shifts in opamps, for example. See fig. 11 in this app note:
https://www.ti.com/lit/an/snoa497b/snoa497b.pdf
Don't believe me? Hold a GSM cell phone near an old transistor radio and call the phone. You'll hear a "burr, burr, burr" sound just before the phone rings. You can also toggle airplane mode. As the phone reconnects to the cell network, you'll hear the interference as well. That said, a cell phone transmits at up to 2 W RF power (+33 dBm). Much higher than the -60 dBm Amir measured.
Wifi is a common source of interference as well. The SSID broadcast will create tons of spurs at multiples of 30 Hz that my APx 525 will happily pick up from any circuit sitting on my lab bench. A metal chassis works wonders for keeping that RF out, though.
RF usually enters audio circuits via the input, not the output as the output has much lower impedance. Not thereby said that RF cannot enter through the output of an audio amp. I recall various HAM radio books explaining how to address RF entry with ferrite rods and/or cores.
Modern consumer electronics are supposed to be somewhat RF tolerant (at least if they're marked with the CE or FCC logos). They're also supposed to "play nice" and not emit or conduct RF out of the chassis.
That said, I find it exceptionally unlikely that a different speaker cable would somehow prevent RF entry, though. That part
is snake oil.
Tom