hallo all,
For me its not possible to Mute/Unmute, Increase/ Decrease volume or briteness with the fn shortcut. 
I try it on my Dell vostro 1720 and on my Samsung NF210 netbook.
My Dell laptop has special keys for Play/Pause/Prev/next and the volume control.
if i try to use the volumecontroll keys nothing happens to.

I m blind and have the need to an screenreader. sometimes a buddy/my wife/ my son mute the sound in gdm (by accident or forget to turn the sound on again).

i have no chance to get the sound back without the screenreader voice.

reproduce:
Boot Laptop (not login)
turn on screenreader in accessiblity menu
mute or decrease the volume with the mouse over the menu
try to change the loudness with fn keys

expect:
loudness change

what happen:
nothing

this is realy not good for blind people :/. 

The same Problem happens in the Lockscreen (login and lock the screen an try to change the sound). i think caused by the same reason.

greetings from germany.