Try to Alt-Tab out of the game so that you see the desktop, change the volume and Alt-Tab back into the game. Maybe that works. I have similar issues with some games on my HP Laptop, I think that's a common problem with the volume "hot keys" on laptops when the function keys F1-F12 have two functions which can be switched with the Fn key. In most games it works, in some it doesn't work.
I can't change the volume using the hotkeys on my dell laptop while playing the game. Anyone else have this issue?