[limin]

How does it work? It sounds to me like it flattens the dynamic range, is this just in my head or is that a fact?

 

[krmathis]

Quote:

Sound Check is a feature that allows you to hear all of your songs at approximately the same volume. You can turn Sound Check on and off in the Effects panel of iTunes preferences. How it works When Sound Check is on, iTunes scans the songs in your library and computes characteristics of their playback volume. As new songs are added, iTunes computes this information in the background. This data is stored in either the "normalization information" ID3 tag or the iTunes Music Library database. The audio data in your music files is never changed. If you encode or "rip" a song with iTunes, the sound check level is stored in the song's ID3 tags. For songs that were encoded with iTunes 1 or iTunes 2, or another application, the sound check levels are stored in the iTunes Music Library database.

Source: iTunes: About Sound Check

 

[blessingx]

Keep in mind Sound Check can be used with third-party applications like iVolume.