My sound/playback device-selector tab shows my fully-video-operative second monitor as "Not plugged in." HDMI output to it works fine, but when this happens, I have to give up on using any sound through this monitor. Problem comes and goes seemingly randomly. I'm far from the only one, according to some fruitless Googling I've done.
MS got me all excited with a Hotfix that was supposed to take care of this precise problem (http://support.microsoft.com/default.aspx?scid=kb;en-us;975538&sd=rss&spid=14498) but as far as I can tell, it was a waste of time to download and install it - I had the same problem almost immediately after restarting.
Anyone got a real working fix or diagnosis for this that's not another dead-end "download the latest drivers" kinda thing?
More about :sound playback devices tab thinks hdmi plugged
I had this problem using an ATI 4670 graphics card on a tv with hdmi input as my second monitor.
The solution I eventually came up with was unfortunately updating to the latest version of catalyst control centre and making sure that the monitor was turned on before the PC main power is switched on. (this also solved the problem I had where the screen was incorrectly scaled if you turned on the monitor after windows had booted up...)