Now I know that I don’t have the best hearing, so perhaps I’m not the best judge. Either way: I don’t like the “Sound Check” preference in iTunes. I’ve been using iTunes for…five years now, and I still can’t quite get the hang of it. From the horse’s mouth, Sound Check is supposed to, “automatically adjust song playback to the same level.” Sounds fun, right? Who wants to struggle through an unusually quiet “I. Introitus: Requiem in D Minor, K. 626” from Mozart’s Requiem followed by an eardrum-splitting “Back in Black” (AC/DC)? What frequently happens is that I’ll enable Sound Check but after a few days, it will seem like every song in my library is muffled. Disabling Sound Check immediately results in a volume boost so noticeable that if the cat were still alive, she’d jump five feet in the air. Leaving Sound Check disabled works well, until my iPod comes across a particularly loud Evanescence song and decides that it simply must blast said track to high heaven. No wonder my eardrums dislike me so much.
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SoundCheck depends on the _average_ volume of each track. If you look at the Summary tab of the Info window, you'll see the number of dB the song's volume is raised or lowered. It doesn't take into account the volume at the beginning or the end of the track, nor the maximum volume. So, no, it's far from perfect.
You could try iVolume - it's a bit more sophistacted in measuring the volume of tracks.
http://www.mani.de/en/software/macosx/ivolume/
Is there any way for iTunes to "forget" its SoundCheck prefs? I'm having the same problem – muffled sound. But it seems that even unchecking the SoundCheck option keeps the music at its new "better" muted level... arrrgh!