Those of you using OS X 10.7 Lion (which I hope is all of you) may have missed the ability to adjust the system volume in tiny quarter steps. This feature was available in Snow Leopard and ditched in the Lion "upgrade." Now, in good news for obsessive compulsive Mac users the world over, the option has returned.
As of the 10.7.4 update, you can now hold down the shift and option keys when you press the volume buttons and the adjustments will be made in steps one quarter the size of the normal ones, letting you fine-tune the sound output. And according to Mac OS X Hints member Arcticmac, the same trick also works for brightness.
As a bonus tip, something similar can be done in iOS. If you can’t get the volume low enough by using the buttons when, say, watching a movie, you can tap the screen and use the volume slider instead. This gives you much finer-grained control over the levels.
Or, you know, just switch to headphones.
Source: Mac OS X Hints.