If your graphics card supports HDMI it will also have HDMI audio drivers allowing the audio as well as video to be carried through it. I would recommend upgrading your graphics card drivers to the latest version and then checking the options within the graphics cards control panel.

Edit: Other thing, since your doing the dvi to HDMI, do the graphics drivers updates and make sure that the adapter you are using has a cross on one side of it next to the normal pins as that's used to transmit the audio.