@natter37 go to your Control Panel and go into Sounds and Audio devices. Make sure that the option "Place volume icon in system taskbar" is checked.

If that does not work you could try uninstalling the sound driver.
Start> Right click on 'My Computer'> Properties> Hardware> Device Manager. Locate your sound driver and uninstall it. Reboot your computer and Windows will install a new driver for you.

If none of the above work for you then you may have a virus affecting your tray. Go here for more information:
http://www.consolediscussions.com/fo...dware-ect.html