There is a local group policy which is blocking you from changing the status of the icons. This site has a registry file which I have checked and is
what you need, download it and run the reg file click yes if it asks for permission and it should be instantaneous but may require a reboot to take
effect.
Right click the task bar, select proprties and select customise from the task bar tab.
Then select "Turn system icons on or off" then select on the the volume control section
edit on 18-5-2014 by NeoSpace because: (no reason
given)