I went through this problem as well, I wasn't using the same codec but volume control had disappeared.
One thing to try is this:
Start -> Settings -> Control Panel -> Add/Remove Programs
There should be a little tab called "Windows Setup" click on the tab. Following this, scroll down to the "Multimedia" option and double click.
Scroll down again until you find "Volume Control" make sure that this is ticked and when you tick it the "Freed Space" bit should turn red, if so that means it is installed on the OS, if it does not turn red, un-check EVERYTHING except "Volume Control" and click OK then click "Apply" and then volume control should be back.
If the freed space icon goes red please post back.
P.S. When I mean EVERYTHING i dont just mean in the multimedia tab, I mean all the sections. sucj as "Online services" and "Acessories"etc.
Hope this helped
