I realize that this thread has been used to poke fun at several responders, but I think that the real issue is - the guy that started this thread has a legitimate question.
The short answer to your question is YES - over time most cables will deteriorate sonically. Before I get a rash of rhetoric back - here's why:
The vast majority of cables for both speaker and interconnects are made from copper. The very moment that the copper conductors are either pulled or cast into wire, the oxidation process begins. Some companies try and prevent this with a litz or enamal coating, which certainly CAN be effective in slowing the oxidation process down, but it the oxidation still occurs, just at a slower rate. Why is this bad? Copper oxide is a terrible conductor - so as the oxidation process occurs, you will notice some loss of high frequencies, followed by some loss of soundstaging ability. At later stages, even bass response is affected, but that would be only in severe cases as lower frequencies penetrate the core of the conductor, which high frequencies travel on the surface.
Silver cables, on the other hand do NOT deteriorate, as silver oxide, while unpleasant to look (tarnish), is still a very good conductor. I always get a chuckle out of guys that clean the tarnish from their silver spades or RCA jacks and then report an improvement in sound - it doesn't work that way, sorry.
Obviously, carbon fibre cables are unaffected by the oxidation issue.
Yes - some cables DO age badly - they don't wear out and it certainly isn't dependent on what music or volume levels are used,but copper based conductors do deteriorate in performance over time.