Enrathi said:
Your red audio cable is in the wrong spot, that spot above the inputs is for audio output. The red and white audio cables should be plugged into the ports to the left of the video cables. Other than that it looks fine.
Listen to this guy, the red audio cable does look to be in the wrong place- it's plugged into the "audio out"- you need it to be plugged into the "audio-in". Also the white cable is not in the "white" plug in- but this should not make a much of a difference other than having your Left/Right audio backwards.
Here is what I would do, turn the Xbox to HD like you already have but I understand you are lacking sound. Keep the game running while you trial and error the audio cables to the back of the TV.
If the sound still doesn't work then there maybe something wrong with your sound board on the Xbox 360- kind of strange if your sound works if you have it set to SD instead of HD...
One more thing- make sure someone has not messed with the TV settings to only allow for Left/Right audio- the red audio should be for the "right" side- however it looks like you have the white plug in the "red" spot, so you will not get any "left" audio output through your TV.
If someone has programmed your TV to only allow for "right" side audio, you will not get any sound- so, plug the white audio cable in the correct spot and go to the TV menu and check audio options.
Let us know if you find a fix.