Windows 7 Unplugging my HDMI display's speakers

I use my computer mainly for living room entertainment and I use my HDTV (LG LED 1080p) as my main computer screen. Everything used to work fine but I've recently started having problems with the sound. When I turn the computer on, the sound works perfectly. However, if I close the TV or change the input from my computer to anything else, and then go back to the computer, the sound is gone.

I looked through the sound options in the Control panel and noticed that the sound problem comes from my TV speakers (identified as AMD HDMI output) being unplugged. This is obviously impossible because the speakers are in the TV so this must be a Windows problem. This output works perfectly when the sound is on.

My TV is connected to my computer through a single HDMI cable for sound and image (ATI Radeon 5700 graphics card and ASUS M4A79XTD EVO motherboard on-board soundcard, both with latest drivers). I use Windows 7 premium home edition 64-bit and I have 4 GB of RAM.

I found that either logging off from Windows or restarting the computer will bring the sound back on. Unfortunately, as soon as I turn off the TV or change the input, the sound goes off again. I am looking for a less bothersome and more permanent solution to the problem. Any thoughts? Thanks for your help!