Copper oxidation can occur but only with the exposed sections of copper. It actually forms a protective coating around the copper core thus preventing further 'attack' by the world of oxygen, sulphur, moisture.
Oxidized copper is often used on historical building roofs for the very reason that it won't decay. It turns a lovely shade of green called a patina.
Although the ends that attach to the binders should be clean, the wire itself does not degrade any further within the core. The film of oxidation is really quite thin, more than ppl realize. Most speaker wire is sold as 99.99% oxygen free, but none of it stays that way for long unless it is sealed in an airtight skin which has been pre-washed in some extraordinarily clean solvents and assembled with the wire in a non-oxygen environment.
Perhaps that is what paying $1000 a foot gets a person? Perhaps not.
Welcome to the wonderful world of chemistry.
