Precisely Adjust Volume Levels on your Mac
You can precisely adjust the volume levels on a Mac by holding down the Shift+Option keys while pressing the Volume Up / Down keys. This lets you adjust the sound level by 1/4 of each notch (as opposed to a full notch), offering precise incremental control to how loud or soft output is from the speakers.
This is perfect if you like to precisely adjust the audio level of speakers, though it’s maybe used best with external speakers which tend to play either too soft or too loud. It works on the Mac internal speakers too, and headphones as well. Easily the best way to find the best volume for you, try it out and find something that sounds good for your speakers and your ears.
You’ll find this great little trick works in all versions of Mac OS X, from the earliest versions onward through Mavericks and Yosemite. Note that newest versions of OS X require a slightly different modifier of the Option key and the appropriate function keys,.
Oh and a bonus tip that many users may enjoy: this neat little precision trick works to change the brightness in incremental levels of any Mac display and screen as well!