Silver plated copper

Riddle me this, Batman. Does the signal travel entirely in the conductor, entirely outside the conductor or part inside, part outside?

That’s what Deoxit and similar products are for as well as contact enhancers like the graphene paste. No big deal. Audiophiles are pretty well versed in electrical contact housekeeping protocols.

This just in! When the surface of a copper wire oxidizes over time the thin oxidation layer seals and protects the rest of the copper wire from further oxidation. Thus, while the effective diameter of the wire is reduced a hair, therefore resistance is increased a shade, oxidation doesn’t appear to be a real issue. 

Explanation: it was installed in the wrong direction. 😬

There is the theory that much of the audio signal - I.e., the electromagnetic wave - travels outside the conductor which, if true, could explain why the skin effect is audible at audio frequencies. 

That link was about as clear as mud. No offense to you personally.