Sound quality degrades after 6 months

[JBMouse]

I have burnt lots of back up music cd's to us in my car, but now after about 6 months some of them sound terrible all of a sudden, with lots of crackle and hiss with the music. Why? They are all low cost, about 20p to 25p each sold on spindles, with no manufacturers name, and I have put full face labels on them all, so what is wrong? cheap disks? label glue reacting or what?
Answers please!!
JBMouse