Have you tried the Sounds applet in Control Panel?
Goto Start > Control Panel, (or Settings > Control Panel in non XP Windows), then double click the Sounds and Audio Devices icon.
Under the Volume tab is there a tick for "Place volume icon in the taskbar"?
I have a Dell Dimension 8100, running windows XP. The sound icon has vanished from my system tray, I believe it is on board sound. How can I get sound back please. I can access the volume control, and all seems to be in order,eg volume up, non mute, Any help would be appreciated.