Headset will not play presumably because of HD cable running from Video Card to TV?

So I have a custom built computer, running Vista Pro (64bit), and it's connected visually to my 42 inch with a HD cable. The HD cable also covers audio. So in the master audio settings it only recognizes the TV. If I plug in my headset (mic/line in) it doesn't recognize it. Is it because the drivers are missing? or because the HD is conflicting. Please help.