What seems to be a new feature in iTunes4 has become more of a hindrance; as I'm playing a song (Three Doors Down's "When I'm Gone" shows this pretty frequently), the volume changes from high to low as an inverse of the song's volume (it tries to make the volume consistent). At times, it's very good, but most of the time, you can hear the audio wavering and sometimes cutting way down then gradually coming back up.
Deleting iTunes' pref file doesn't help, neither does unchecking every sound enhancing feature. I even copied a new version of the song, locked it immediately, and tried playing it to make sure iTunes wasn't just editing the song file. The only fix I've been able to find is to turn off the equalizer. iTunes3 didn't show this behavior, so unless you use the Music Store, you might want to just keep using iTunes 3 instead.
I'm going to hunt around in the plist file to see if there's such an option for the sound balancing, but it seems that turning off the equalizer (which I need to use for iTunes to make half-decent sound) must be turned off.
[robg adds: I haven't noticed any sound fluctuations, but then again, I'm far from an audiophile and usually don't play iTunes at a loud volume, making it tougher to discern any volume fluctuations. I have, however, heard similar reports as this, so I thought I'd post this to see what others had to say on the matter...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030502114441595