Apple Magic Keyboard

Recently got a new magic keyboard for my iMac, but i can't figure out why the function keys operate differently as advertised, i have found the option to rebind brightness to F1 and F2, as well as seeing all windows to F, but cannot rebind volume up/down/mute to F12/F11/F9 respectively? is there an option to return the function keys to their listed purposes? for example the F11 was bound to "show desktop" instead of volume down.