On keyboards with volume controls, the system emits a small "beep" when you use those keys to set the system volume. Over in the comments to the hint about using shift-arrow keys in QuickTime previews, percy commented that the shift key has a pleasant side effect when used with the keyboard volume controls -- it silences them! If you hold down shift prior to increasing or decreasing the volume, you won't hear the small "beep" noise. Wonderful...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020608113514758