A strange situation indeed. I have the volume slider available in my digital output settings, but I have Realtek AC'97 audio as opposed to the HD audio which you have. It could be an issue that needs addressing by Realtek themselves, in which case I suggest that you email them to see what they say. You can find the various email addresses on the following page (the one you require is under the heading 'Computer Peripheral Products').
Dwarf
Realtek