Volume buttons not working in 14.04

Looks like a known bug on Launchpad.

According to the thread, the current fix is to...

1. Open terminal (Ctrl+Alt+T)

2. 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.