Adjusting the volume on your Mac via the keyboard is pretty easy: there are 3 buttons, one for Volume Down, one for Volume Up, and one for Mute.
When you adjust the volume, a sound is played to help you determine when it is loud enough or quiet enough. Most times I find this helpful, but sometimes I want to adjust the volume without hearing the noise. To temporarily disable the "volume change" sound, hold down the shift key while you adjust the volume. You will see the visual indicator on-screen, but no sound will come out.
For example, on my iMac keyboard, I can increase the volume by pressing fn+f12, but if I press fn+ shift+ f12 it will increase the volume without making any sound. Very handy especially if you are in a meeting or working in a library, etc.(Thanks to TUAW reader @webgalpat for that tip!)
But what If you never want to hear the sound when you adjust the volume? Go to "System Preferences.app" and then click on the "Sound" preference pane, and UNcheck the box next to "Play feedback when volume is changed" (That tip and others were covered in our recent 5 Tips for Switchers article.).
Now let's combine those two tips: what if you have changed the preference so that the sound is not normally played when you adjust the volume but you decide that just this once you want to hear the sound level changes? In that case, just hold down shift while adjusting the volume and it will make the noise.
Bonus tip: you can use the keyboard to adjust the volume of your Mac, or mute/unmute it, even when the screensaver is on -- without entering your password or disabling the screensaver.