Sound quality degrades after 6 months

[EMPiRE]

Most likely the quality of these "low cost" CD-R's is plain bad... If there are too many errors on the CD-R the error correction can not fix this anymore and this results in crappy sound.

If you are also using these CD-R's for data then you get too see errors much faster than on Audio CD's

look here (a bit outdated but still valid):

http://cdmediaworld.com/cd_quality.htm