Q: I’ve always wondered, why are mic, guitar, and line signal wires shielded, but speaker and power cables aren’t? It seems like it would make sense to at least shield the power cables?
A: Line, mic, and guitar cables are shielded because those signals are greatly amplified before they are converted back to sound waves; any noise that those wires pick up along the way will also be similarly amplified and become audible. AC power is never amplified, and speaker-level signals are already amplified before they pass through speaker cables.