Using keyboard controls to change your Mac’s volume works in Mac OS X 10.5 just as it has in previous versions: Tap the volume up button to get louder, and tap the volume down button to get quieter. If the steps between volume levels are too big for you, however, Leopard has a fix once you know the magic key combination.
If you hold down Shift-Option while changing the volume, your Mac will add four smaller steps between each major step. If you need to use the fn key when adjusting the volume, don’t forget to hold down that key, too. 
Use Shift-Option to fine tune volume adjustments.
This is an easy way to overcome too loud/too quiet problems when changing your Mac’s volume without having to touch the volume control on your external speakers.