I've experienced this problem many times in the past. The first time I saw it, the speaker cable that was involved was actually Monster cable.
One source of chemicals that corrode copper can be the plastic insulation of the wire itself.
Wire insulation on wire fits pretty tightly and is itself usually pretty air tight. IME the corrosion is limited to just the ends of the wire, and maybe a fraction of an inch under the insulation at each end. The wire in the middle of the cable usually remains pretty bright. Therefore, if there is corrosion the length of the wire, the insulation itself is probably the cause.
That all said, most corrosion is very superficial. I've soldered wire that was oxidized this way by cleaning its surface with fine (wet/dry) sandpaper. As you probably know, corroded metal does not take solder, so the ability to clean it up with fine sandpaper and solder it is strong evidence that the corrosion was very superficial.
I've also done electrical tests of wire that was superficially corroded this way, and there were no measurable differences, provided I was still able to make a solid electrical connection to the wire itself. Generally the connectors will be tight enough so that the superficial corrosion does not reduce their conductivity.
Bottom line, don't worry about superficial corrosion unless you want to reuse the wire and either solder it or install different connectors. If you are soldering it or changing the connectors then clean the wire carefully with very find sandpaper - maybe 150 grit or finer.
I have seen electrical tape with sulfur in it (self vulcanizing rubber tape) actually corrode fine strands and eliminate or signfiicantly reduce conductivity.