[ubuntu] How do I get my volume keys working?

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http://ubuntuforums.org – My laptop's media keys (volume up/down/mute) are detected via `xev` and `showkey`. I can go into System Settings > Keyboard > Shortcuts and set the volume controls with the media keys. However when I try to use the keys they do not function properly. I tried setting the keys and rebooting the system but that hasn't worked. Any ideas? System Info:Dell Latitude E6500 Ubuntu 12.04.1 (Hardware)