The ATI driver you mentioned is for the HDMI only. The realtek is for analog and possibly optical/coax.
Are you trying to use the analog ports? (they look like 1/8" headphone jacks)
If so, there is a second audio chip that will require a driver to be installed to use (which it looks like you do) and you also need to go into the Windows sound properties in Control Panel and select which audio out device you are using.
My guess is that you're trying to use analog, but you have the OS set to output over HDMI or digital.