Charles, it would be extremely unlikely that any home environment would have electromagnetic fields strong enough to significantly affect the relatively strong signals that are traveling along a speaker wire into the low impedance of a speaker, so as you note, for all practical purposes speaker wiring can be said to be immune from interference. Shielding therefore isn't required to protect the signals traveling to the speaker.

The signals traveling along an interconnecting cable, by contrast, are about a thousand times weaker than the amplified signals going to the speakers and can be affected by interference in the home. So, coaxial cable which has an outer shield to contain the signals and protect them from outside fields is typically used and is generally satisfactory. Some professional applications where especially high interference over long distances is encountered use instead a balanced connection setup which employs a different method to cancel outside interference which impinges on the cable.

-----------------------------------

Enjoy the music, not the equipment.