What happens is when you connect the HDMI to the TV a new item pops up under sounds configuration in control panel. Remember HDMI is used as it passes both Video and Audio signal to the TV. Previous to HDMI you needed multiple cables (one for sound and one for video). What I noticed under sounfds where it showed my soundcards whenever I was connected to the TV a Shortcut ATI would popup. My laptop uses an ATI graphics card but because HDMI uses both video and audio the description of the device was ATI. I have a realtek sound card and I noticed it was set as default when the HDMI cable was attached. I clicked on the ATI shortcut and I had sound through the TV. When the HDMI cable wasn't connected the ATI shortcut wasn't there.
So what you need to do is go to sounds in Control panel and look for the part where it shows your sound card before you plug the HDMI to the tv. Then after you find the card plug the HDMI cable in and a new shortcut should appear. That is the one you need to click. You don't need to change the default because you want your soundcard to be the default when not pluggedi into the TV.
I have a GTX570 which I use to connect my PC to my TV via HDMI. For two years or so everything has been fine until I recently returned from holiday and the picture still works but there is no sound. I have googled the problem and the usual answer is to set the TV as default in audio properties. I had already done this and it is set as default (shows full name of TV).
When I click test the green bar goes up and down to show sound is being played but it just doesn't come through the TV. Another solution I have found is putting a cable between the sound card and the graphics card through SPDIF but this doesn't seem to be required as it has always worked without this.
Please can you help I am pulling my hair out.