Everything ok with this keyboard except one thing.

Are you trying the volume control at the media controls? See here.

Also check to see that your Operating system volume control is on and up.

good luck.

I had a similar problem with a microsoft wireless keyboard. Tried all manner of fixes including updating a HID Serv DLL file, none of it worked (either mute or volume would work but not both). Eventually I stopped using the single USB connector to connect the wireless transmitter unit to the PC. Instead I connected the wireless sender unit to the PC via the Purple PS2 connector (keyboard) and via a Green USB - PS2 adapter (mouse). Don't ask me why but all the buttons started working straight away.

Hope this helps