There exist four different versions of "Dark Side Of The Moon". The original version is the mix that appears on the original LP, and all CDs made before 1992. In 1992, for the album's 20th anniversary, the music was remastered, and this mix has been used in all subsequent CD pressings. In 2003, for the 30th anniversary, the album was remastered again for release on a hybrid SACD. In addition, there was a Quadraphonic LP version of the album.
In addition, each remaster has seen changes to the album's famous cover. The original drawing of a prism and light was changed to a more photographic style for the 1992 release. For the 2003 version, the album cover was redone in a stained glass motif.
The differences between versions are highly noticable. In the 1992 remaster, the various voices that appear in tracks are mixed lower, making them harder to hear. On the LP version of The Great Gig In The Sky, for example, one can clearly hear "I never said I was afraid of dying", while in the 1992 remaster, the voice is barely a whisper. The 2003 version is mixed in 5.1 Surround Sound, which changes the whole sound of the recording. The sound is much less focused unless your sound system is set up right. In addition, more attention was spent bringing out little details in the sound than creaing a full sound. Furthermore, because the album was mastered from the original tapes, the sound loses a bit of power because of tape degredation.
Among Pink Floyd fans, the best mix is the Quadraphonic mix, but for average fans, it is best to stick with the 1992 remaster. It can still be found in essentially every music stores.