What this really refers to is gnome-volume-control-applet which is what controls the volume in the panel/indicator applet.

In Ubuntu 10.04 it is just the applet mentioned above, run that from the command line and it will start.

I lost mine when I removed pulseaudio trying to fix audio issues with Flash and Opera.  It also removes the gnome-volume-control-applet from starting.  *I also note that reintsalling pulseaudio did not bring it back, it seems only an OS reinstall or manual intervention would bring it back.

You need to add it to your startup applications to make sure it comes back after a reboot.