gammajo has made the observation I have been waiting for: I too have modest quality 10G fine-strand copper speaker cable with clear sheathing. Within 5 years, green oxidation is evident over the copper's surface throughout the length of the cables.
The question for our group is this: if we keep our connection (exposed) ends clean, does the fact that the surface of the sheathed strands is oxidized matter? randy-11 makes a point: is the core oxidized? I would say 'no', but isn't it true that signal transmission takes place primarily on wire strand surfaces, so who cares about the "core"?
The fact that our sheathing is clear should not matter (light certainly does not cause this oxidation...)...so one might think that this happens to opaque sheath cabling as well...
So, electrophysicists weigh in: does oxidized fine copper strand speaker cable (with clean connections) impede signal transmission significantly? Thanks.