I think it would help to explain my setup first to help understand my problem. I'm using a Samsung T220HD monitor (which has built-in speakers) which I have both my computer and my xbox 360 plugged in through hdmi ports. I'm using a pair of Turtle Beach X12 headphones which when I use my Xbox 360 I have them plugged into the headphone jack on my monitor. When I'm using my PC i usually just plug my headphones straight into it.
However, sometimes I'm playing games on my xbox (with my headset plugged into the monitor) with my pc on to skype with friends or quickly look up a video, and today for some reason my computer no longer recognizes my HDMI audio output. I did not download any drivers or anything like that. It was literally working one minute and it stopped working the next minute. I tried turning off the computer and unplugging my headphones from the monitor and audio was coming out of the monitors speakers, but as soon as I plugged in my headphones, that caused the audio to stop working again. It's not the port or the headphones themselves I believe because the headphones still work fine and the audio from the monitor's headphone jack still works when I'm using it with my Xbox 360.
I looked on my computers control panel and under sound it shows "Realtek HDMI Output" but it says its not plugged in. It's obviously plugged in as I can see the picture right now.
I tried switching the ports the HDMI cables were plugged into but that did nothing. Unplugging the headphones from the headphone jack also does nothing.
Try updating or rolling back your video card's driver. Other then I don't really have an suggestions.. I had this happen to an old laptop of mine. Never got fixed but you might get lucky. It may be that the hdmi port is messed up for whatever reason and no longer works fully.