I came by this on lifehacker.com.

I often felt that volume adjustment offered by the volume control keys on your Apple keyboard didn’t adjust the volume to exactly where I wanted it and often supplemented the adjustment with turning the volume slightly up or down on my external speakers or headphones.

Turns out Apple thought of a solution to this already.

Simply hold down the SHIFT and OPTION/ALT keys at the same time as you use the volume keys an you will get 1/4 increments instead of a full block.

Awesome.