My Inno3D Ion needs to install 2 audio driver. One for HDMI (Nvidia HD Audio) and the other for computer 3.5mm headphone jack (Realtek) audio out. I do not use SPDIF out as the HDMI audio is good enough.
HDMI pass-through audio activates only when playing movie in PowerDVD. However, sometimes the second sound driver needs to be disabled in the control panel, in order to make the HDMI pass the digital signal to my amplifier.
Normal windows sound goes through the 3.5mm audio out.