I don't know if you want to make this a hint or not, but the balance adjustment in the Sound Preference Pane is definitely drifting away from center for some users. I wrote in about this a couple weeks ago, but without any confirmation of what was causing the problem. No one yet knows exactly what is causing the problem, but it is affecting other users. This weblog contains a discussion about the issue.
If there's a hint in here, it's that if you notice the sound quality on your mac deteriorating, check the balance adjustment of the Sound Preference Pane. It's probably skewed to one side or the other. Especially if you use a PowerBook and often use the keyboard keys to adjust the volume. No one yet knows if this is a bug in iTunes, or in OS X, or exactly what triggers it. As for me, I've already filed a bug with Apple, as I hope many others have as well.
[Addendum: This hint originally ran on July 16th, 2003. I'm re-publishing it with additional information supplied by macosxhints reader kngofwrld:
While searching through the Sound Preferences Pane I noticed that my sound output balance was off center. I set it back but after using the computer for a few days I checked it again and it was off again. Checking Apple's Knowledge Base, it turns out that this is a real bug (article number 25466), and can occur when the processor is under a heavy load and the volume controls are pressed rapidly.Apple's explanation for the problem's cause explains why I haven't seen the bug myself -- I don't use the volume keys; I just tweak my speakers' output setting.]I didn't realize that my sound balance was off center, even using headphones. But now that I have set it back to the middle I get much better sound.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030714025520659