There are several ways to adjust the volume level in OS X. These methods apply to the entire system volume, not just individual applications. More preferences can be changed using the Sounds section of System Preferences.
The first method of adjusting volume is the sound icon in the top menu bar, upper right-hand side. Clicking on the sound logo will bring up a slider. Drag it up and down to adjust the volume.
There are also three physical volume keys on the Apple keyboard. usually located on the F10-F12 keys. These will raise, lower or mute the volume. When pressed, a volume box will temporarily appear on the screen to let you know where the volume is currently set.
Using the physical buttons will increase or decrease the volume one whole square. If you want to adjust the volume incrementally, hold the Option + Shift keys while tapping the physical buttons.
As you may have heard, adjusting the volume with keyboard buttons will cause a “click” with each press. This can be silenced temporarily by holding the Shift key while adjusting volume. To permanently mute the volume notification sound, open System Preferences in the dock or Applications folder.
Under the Hardware section of System Preferences, click on the Sound icon.
In the Sound preferences window, click on the Sound Effects tab.
Towards the bottom of the window, uncheck the box next to Play feedback when volume is changed.
This will make sure that no sound is made when adjusting volume from the keyboard.
My F10 key, instead of lowering the volume clears the whole of my screen. Pressing it a second time reverses that. The F12, instead of raising the volume makes the calculator et al. appear and disappear. The F9 doesn't mute the volume, but it doesn't do much of anything else either. What the….?