Right- sometimes, the cart is placed ahead of the horse.

I was referring to induced current being a problem, often caused by cables in close proximity and parallel to other wiring that may be conducting much higher current and/or voltage. I did a job in the late-'90s where the audio system would make a loud snapping sound if the level from the mixer reached a certain level. It was a 70V system with previous wiring woven through other cables without any care about how that could affect things.

WRT "Whew! I'm out of time at the moment, maybe someone will follow on and explain why "balanced line" is so helpful when dealing with and preventing noise issues. And, why that missing cable system "ground" can be so bothersome. ",

In addition to what Ernie posted about CMRR, and that one aspect is the main reason true balanced lowZ circuits are less noisy, the audio circuitry and chassis aren't at the same potential in a true complimentary balanced system- the audio is usually floated ~50 Ohms from the chassis and the shield is only supposed to do what the name implies- shield the audio or video from stray noise. Devices using unbalanced signal don't even need more than the center conductor to use the signal if they have polarized power plugs and the chassis is connected to the neutral or if the chassis are making good contact. Look at the shorting jumpers on most of the integrated amps from the '70s-'90s- it was a piece of wire, bent so it could be inserted in the Pre Out and Power Amp In jacks. No shield whatsoever and no noise was introduced, either.

Also, EMI shielding can be used, but it's a bit less certain as to whether it will work when it's protecting cabling. Speaker magnets, tape heads and things like that can be surrounded by something like Mu-metal but that's not a good solution for cabling because it's stiff and the edges are very sharp although it would be a good thing if they have come up with a way to make this in a foil but I'm not aware of it. Another way to protect cables is to use copper tape with stainless braid (looks a bit like screen material) and grounding it at both ends, assuming the ground points are equipotential. If they aren't, it would be grounded at the end with the sources and preamp.

EMI was more of a problem with car audio in the late-'70s and early-mid '80s. I think it was the Chevy Celebrity that had a service bulletin for re-routing the main heater wire because it passed behind the head unit and would induce noise through the tape head. The Cavalier had its own bulletin for the hood bolts- the OEM part was coated with a material that didn't stop the hood from being bonded to the chassis and when body shops would replace the hood or its bolts, they often used the less expensive oxide-coated bolts, which provided electrical resistance and therefore, allowed noise to exit the engine compartment.