Accidently I've removed the Gnome Volume Control while trying to remove an applet nearby from the GNOME main menu panel. Unfortunately in GNOME 2, I couldn't find a way to to return back (restore) Gnome Volume Control to the main panel. After a bit of pondering, I've managed to find a way.
Here is how I managed to restore it back:
1. Navigate to:
System > Preference > Startup Applications
2. Click on Add, then add and type the following:
Name: Volume control
Comment: Launch volume control applet
Adding the gnome-volume-control-applet will launch it every time a new gnome session (with the same user) is initiated. On next gnome login you will see the icon to appear again in the notification area. Cheers 😉