Did you check "Show disconnected/disabled devices"? (right click in Sounds > Playback)
According to Nvidia:
No, you do not need to connect an S/PDIF cable from your sound card to the Geforce G210, GT220 or GT240 graphics card or any newer version which has its own internal HD audio codec. NVIDIA has added an HD Audio codec right into the graphics processor of newer graphics cards therefore no longer needing the sound card to provide audio when a user is connecting the graphics card to an HDTV via an HDMI cable.
Also, if you have time, could you check if you have any "NVIDIA High Definition Audio” devices in you Device Manager? I have four! Seems a bit much, bu maybe its normal?