NVIDIA video cards don't have an audio chip unlike the ATI cards 3xxx,4xxx,and 5xxx cards that have realtek sound chips built-in. NVIDIA use a PASS-THRU approach or technique. The sound card SPDIF connector must be connected to your NVIDIA card. Nvidia cards have SPDIF connector to take in audio and link it to the DVI/HDMI connector. That will facilitate audio link from your PC->SoundCard->Nvidia->HDMI->HDTV.
I have a BFG 280 OC on one of my PC. I'ts connected to a 52 Inch HDTV through HDMI(Audio and Video). How it works and what HW connections i made is on this thread... Read it please.