If you’re using an Apple keyboard (or many third-party keyboards with a dedicated audio volume button), you can adjust the volume up and down with the push of a key. For many people, that’s enough.
For those who need a little more control, you can adjust the volume in quarter increments (rather than whole increments) simply by holding Option + Shift keys while pushing the Volume Up or Down keys. You’ll notice on the Volume bezel that appears on screen that the volume is adjusting a quarter box at a time, rather than a whole.
And for those of you who are annoyed by the quacking/beeping/burping sound with every press of the volume key, you can temporarily mute that sound by using Shift+Volume Up or Down keys. Unfortunately, you can combine the two to make silent adjustments in small increments.