Why do some CDs sound better than others?

All CDs have to have an original source recording. The overall sound quality depends upon how good that source recording is, and how well it has then been mastered on to the CD. This is where the letters AAD or DDD printed on the CD, have an effect on the sound quality, so an AAD has two Analogue processes before the final digital process, it will suffer from a little more hiss, caused by the analogue tape hiss, but overall have a more natural analogue sound. Also if the CD has double the amount of tracks usually found on a CD then this usually means the music has been compressed to squeeze more on the disc, these discs like most compilation discs will always sound poor.