I run Ubuntu 14.04 on a Sony Vaio CS series laptop. Normally, the volume is controlled by pressing Funcion+F2, F3, and F4 for mute, volume down, and volume up respectively. I accidentally reassigned the volume up keyboard shortcut to a different key. I tried resetting the keyboard shortcut by going into the keyboard shortcuts editor in Ubuntu and pressing Function+F4, but that didn't work. My mute function and my volume down functions both work flawlessly, but volume up doesn't work at all. Please don't answer telling me to change the keyboard shortcut in the Keyboard Shortcuts manager, as I have already tried that. Thanks for your help!
I found a bug report on launchpad regarding this issue and it seems when reassigning the media keys some prefix is missing. The following lines are somewhat of a summary of what is discussed in bug report
You can issue the following command to see your current keybinding:
Output should be
You can either reset them via either:
Of course the latter is more specific and might not work with all systems out there but I I was happy to find out it's just the string
For a list of possible keys to map you can use
If your setup is different you might want to figure out your media key's name and set it via dconf. I haven't done it myself but it is mentioned in a comment which in turns links back to another askubuntu-question: Where are GNOME keyboard shortcuts stored?.
Edit: Couldn't help but to play around with it a little bit and as my keyboard is lacking some Previous/Next buttons I found CTRL+Volume-Down/CTRL+Volume-Up useful: