I've just built a computer specifically for use with my new HDTV (Sony 42", 1080p).
As I understand it, there are two ways to get sound to the HDTV...Neither of which are working atm.
1) I have a video card that has a built-in HDMI-OUT port on it, and a SPDIF-IN on the board itself. I've plugged the SPDIF-OUT from my on-board sound into the video card. The idea being that all I would need to plug into my HDTV is the HDMI cable, with the sound running along it like normal.
But I don't get any sound.
2) One of the TV's HDMI ports has two-channel RCA audio jacks next to it. I can plug the HDMI cable in without any sound plugged into the video card, and run an audio cable from the sound card's 2.5mm jacks into these RCA jacks.
But I still don't get any sound.
Here's something odd too... When I run the sound through the RCA jacks, I can get sound if I unplug the HDMI and leave the sound plugged in.
I also got it to work once by switching back and forth between "HDMI-HDTV (Audio Enabled)" and "HDMI-HDTV (Audio Disabled)" in my video card's driver settings (Nvidia). But It only worked once and I can't get it to come back.
The TV doesn't seem to have any option of selecting where I want the sound to be coming from.
I've also tried using a DVI-HDMI adapter and the 2.5mm jack and it still doesn't work.
I have plugged headphones into the 2.5mm jack and I get sound every time...
OK I got something to work finally. By unplugging and disabling the SPDIF, and disabling it in windows, then disabling, enabling and disabling again the audio via HDMI, I got the 2.5mm jack to output sound to the TV.
I think by the time I'm ready for any surround sound with this thing, I'll get a new sound card that will probably do all of this properly...
Only problem is that every time i reboot the computer, to get the soudn to work i have to enable sound through HDMI, then disable it. Then it works. Sort of a pain.
It seems that any time the signal to the TV changes at all, the sound quits working and i have to enable/disable the video card's audio in order to get it working again.
The problem is that if I am playing a game, as soon as the game starts it goes to full screen which makes the audio stop working. I can alt-tab out of the game and do the enable/disable trick to get it working again but as soon as I switch back to the game it stops working agian.
This is obviously a problem with the TV not being able to correctly decide where to pull it's audio from.
Going back to your original post, it seems that if you unplug the HDMI cable you then get the stereo sound over the RCA inputs, correct? If so, then check your TV setup to see if you can program that particular HDMI input for video-only, so that it expects the sound from the RCA inputs. For example, my Pioneer plasma TV has an HDMI input for PC with a couple of RCA jacks for the audio, and I can set that particular port to accept audio from the RCA jacks.