Mac: When you hit the media keys to change the volume in OS X, you hear a somewhat obnoxious popping sound that is supposed to help indicate the current level. This might be helpful the first time your turn on your computer, but it's pretty annoying in the long term. Thankfully, Cult of Mac points out that it's very easy to turn off in most versions of OS X.
All you need to do is head into your System Preferences and click the "Sound" option. On the "Sound Effects" tab, uncheck the box "Play feedback when volume is changed", Now, when you change your volume with the keyboard you won't hear those popping sounds anymore. If you're looking for a more temporary fix, just hold down the Shift key when adjusting the volume.
Turn Off That Annoying Volume Popping Sound in Mavericks [Cult of Mac]