It's a combination of all things mentioned here.
Source material has the biggest impact on the sound followed closely by mastering and by the vinyl quality and packaging control.
As for the earth's magnetic field slowly deteriorating the tape - not true. You might get some print through but most tapes prior to 1974 hold up extremely well. It's all tape recordings made between 1974 and 1986 that suffer the most from deterioration....and yes...that includes "Hotel California" by The Eagles. Why you ask? Well, all tapes made prior to 1974 used whale oil as a lubricant. In 1974, the manufactures changed from whale oil to synthetics. Not knowing any better, the figured it was just as good...and it was but only in the short term. After a few years, the synthetic oil breaks down rapidly making all of these tapes unplayable.
35mm sound recordings also fail in the longevity test. A lot of those great hi-fi 35mm magnetic recordings have turned into vinegar because they were stored in metal boxes. However, if they were stored in cardboard boxes...they should be fine.
Edited by LFF - 9/18/11 at 9:27am
