you should see a label on the back of the tv which says (rgb/hdmi-pc audio) its hard to tell from pics what kind of plug that is. you might have to get a headphone jack to rca adapter or if you have a home theater just hook the optical from the board to it. The tv optical is for out only.
I tried that already it doesn't work the tv is set up to to get the audio and video through the hdmi inputs. i cann't split it up.
your only option right now is to use the pc port on tv not hdmi unless you upgrade video card to one with a hdmi port.