Kyle Weber tweeted a nice little tip for adjusting your Mac’s volume by quarter increments:
option shift volume for a more precise volume control.
So, try it out: hold down the Option key (⌥), the Shift key (⇧) and press the increase or decrease volume hardware button on your Mac.
On my older MacBook Pro they are F4 and F5 across the top of the keyboard, but on newer Macs or other models they may be elsewhere.
The symbol on the keys is the ‘speaker’ symbol you see in the screenshots here. The Mute key has no ‘rays’, the decrease volume key has one ‘ray’ and the increase volume key has 3 ‘rays’.
With the Option and Shift keys down the volume increments by quarter units, rather than whole units.
When I change the volume the usual way a transparent grey box appears on screen and shows white blocks filling black squares across the bottom, below the speaker symbol. One key press adds or subtracts one whole square.
When I hold down Command and Option as well the white boxes are only ¼ the width and the volume changes by ¼ of a unit.