Attached Link: http://www.hyperion-records.co.uk/bronzed.asp

If the bronzing is bad enough, the laser won't reflect properly and the data will be misread, at least sometimes. If the aluminium layer is lost completely then that part of the disc will become unreadable. This is progrssive - once the air can get in and start oxidising the disc is doomed (unless you can reseal it). Usually the bronzing starts at the edge and works in; if your discs are uniformly yellow it may just be the lacquer aging. In any event, I'd back up to CD-R(s) and contact the publisher: when I had a dozen discs go bad, from two different companies though the pressing plant was the same, they arranged for free replacements: I've attached a link for one of the publishers which gives information on the problem.