The volume customer keys of my Mac don't work anymore I used to press F10 to mute, F11 to decrease volume and F12 to increase volume, but all of them suddenly stopped working. If I press any of those keys now, they send me to another screen that has nothing to do with the sound volume.  

How can I fix them?
Basically


Open System Preferences
Select "Keyboard"
De-select "Use F1, F2, etc., keys as standard function keys"