The standard F1 through F12 function keys in OS X by default are set to change a number of common system settings, including audio volume, backlight and brightness levels, and media player controls. Convenient as this is, sometimes their default behaviors can be intrusive or too crude for your needs.
For example, if your system is connected to an external amplifier that is set to a high volume, then the increments changed by the volume function keys may be too crude for your needs, and the corresponding audio indicator pop may be unpleasantly loud and intrusive, especially if you are listening to a soft piece of music.
If you find this to be the case, then there are some quick ways to effect a finer degree of change for these settings, silence audio indicators, and even get quick access to corresponding system preferences.
Silence the audio volume indicator
When you adjust the volume through the function keys the system will play a small pop sound; however, you can prevent this by holding the Shift key when adjusting the volume.
Finely adjust settings
All level-adjustment function keys, including those for keyboard backlight, audio volume, and screen brightness, can be more finely adjusted by holding the Shift and Option keys while pressing the function keys.
Access system preferences
While holding down the Option key, if you press a function key then its corresponding system preferences pane will be opened. For example, to open the audio system preferences you would hold Option and then press either the volume up or the volume down key. The same goes for backlight and brightness controls, but also for some of the other function keys like the one for Mission Control.