"Robert L. Bass" said:

I disagree. You will not get audible hum from defective speaker cables.

Depends on the circuit.
There's a feedback loop in most amps from output to input, and said
input is out of phase with the source signal.
Hence, there's no attenuation from feedback for signals intruding in
the feedback loop. Because of the high open loop gain in that case,
hum and other undesired rubble may well be audible.
It can even be demodulated, in case of strong RF signals.

I've actually seen amps humming and even oscillating due to
interference that came in through the speaker outputs.

Some small caps across the outputs and gnd, as well as chokes in the
output circuit may well prevent this, shielded speaker cables may help
too.

--
Sander deWaal
Vacuum Audio Consultancy