I recently built a new HTPC for watching movies on my HDTV. The motherboard I selected was a Gigabyte MA785GM-US2H with built in ATI Radeon HD 4200 video card. Below is a link to the device.
Gigabyte MA785GM-US2H Motherboard - AMD 785G, ATI Hybrid CrossFireX, PCIe 2.0, USB 2.0, RAID, HDMI, DVI, VGA, AM2+ at TigerDirect.com
The computer has both a DVI and HDMI output on the rear pannel, as well a the standard line-out audio jack. My television has two HDMI inputs, one is just an HDMI input, the other is an HDMI input in tandem with a dual channel audio input. I have tried connecting my computer using just the HDMI cable to the one input, and have tried connecting in in conjunction with an audio cable to the other input set. In both cases I get no sound. However, if I connect the computer to the television using the DVI input and single channel audio, I get video (not HD of course) and sound. I have also tried connecting just the dual channel audio cable to the HDMI/audio input set, but not the HDMI cable. I can get sound in this case, but obviously no video.
Any idea why I am unable to get both HD video and sound? It's as if my television is trying to receive the audio via the HDMI cable, but my computer is trying to send it via the audio cable, and the result is no sound when both are plugged in.