In my Xubuntu 12.10 install, using the multimedia keys doesn't actually have any effect. Even though the OSD shows changes in volume, the actual volume doesn't change. The same thing applies to the mute button. I don't want a specific application's volume to be changed - I want the system-wide volume to be increased/decreased/muted.
I looked at threads such as this one, but they didn't help. Also, I'm seeing this issue on a system-wide basis, not just in certain situations, like when playing YouTube videos.