You might assume you have a bad drive or the SATA interface/cable is bad, or the power supply is bad/weak to the drive. These are all possible issues, but definitely check your SATA cable for "twisting". It is a big issue because until the error stops or times out, your system will not boot (in my case this was the case even though the drive with the issue was not part of the OS or booting process at all).
If you run an open rig that you move around often that ha........
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. *Ialso note........