Volume buttons not working in 14.04
Looks like a known bug on Launchpad.
According to the thread, the current fix is to...
- Open terminal (Ctrl+Alt+T)
Type the following three commands to reset the volume/mute keys on your keyboard...
gsettings reset org.gnome.settings-daemon.plugins.media-keys volume-up gsettings reset org.gnome.settings-daemon.plugins.media-keys volume-down gsettings reset org.gnome.settings-daemon.plugins.media-keys volume-mute
It worked for me, hope it works for you too.
If resetting the keyboard shortcuts didn't work (such as in my case) move or delete the .gconf directory:
cd && mv .gconf .gconf.old
Logout and log back in again.
Additionally, to fix the mute button, execute
gsettings reset org.gnome.settings-daemon.plugins.media-keys volume-mute
from a shell. Worked for me, hope it helps you too.