Hah, that AM-receiving cable was at a theater and only a couple hundred feet long.

In general, with low-level analog audio and non-ridiculous lengths, additive noise effects are likely to become audible long before attenuation or especially frequency-dependent attenuation. As a decent heuristic, as long as the DC resistance is small compared to load impedance, the cable impedance is unlikely to be a problem. For the connection from the amplifier to the speaker, additive noise is unlikely to be a problem, so the DC resistance is even a decent heuristic: keep the round-trip resistance below half an ohm or so and you should be fine with most speakers.

I used to have a fairly nice Rotel preamp that would receive CB radio transmissions sometimes. Whatever it was that was behaving as an AM detector must have been near the output of it because the intensity was unaffected by the volume control.

That was almost certainly the result of illegal transmitters, but it was annoying. One time I heard a man shouting through the stereo and the signal was hot enough in RF land that it made my X10-controlled lights flash on and off.

Later, I got DirecTV and it became differently-annoying: The noise of the satellite receiver switching bias voltage to select between different LNB polarities was sufficient to make a loud pop through the speakers (again, unaffected by the volume control). This made channel surfing very noisy. I was able to reduce it rather substantially with some very deliberate choices in audio cabling construction.

But with better gear (and with the differential[1] signalling that every avenue of pro audio seeks to use by default), this kind of stuff is usually a complete non-issue.

[1]: We popularly call this kind of connection "balanced," but we're wrong about that since there's usually hardly any concern about impedance matching. But it's definitely consistently differential, so I find this less-popular nomenclature to have the right amount of specificity to ~fit reality.