You're right Daniel. It doesn't need to be moist for this issue to happen. Although moisture will accelerate oxidation, it's not likely contributing to your problem.

Many companies jumped on the bandwagon for the fat Monsteresque clear coated speaker wire, and they didn't put too much time into engineering of the jacket. The decomposition of the jacketing is the cause of the oxidation, or 'greening' of the copper. As long as you have good contact at the amp and speaker ends, the signal will still pass. Over time, it will get worse.

Get some new cable.