Check in your manual for the video card. Does it say that, to get sound output on the HDMI connector at the back, you need to connect a small cable from your mobo's sound port to the video card? Some video cards do not handle the sound directly themselves - they merely allow you to feed the existing sound signal through them to the HDMI connector.
If that is not the case, does your video card say it also acts as your sound card? If it does, do you have to install a sound driver for it? And then , do you have to set Windows' Sound controls to use that sound card source, rather than another one?