I was trying to connect my Xbox 360 to my shiny new Sony Bravia KDL-46EX720BAEP Flatscreen. I thought it would be sufficient to plugin a HDMI cable and I would be all set. The screen was immediately detected, but I heard no sound from the console. It took me a few minutes to find this awesome hint which allowed me to hear sounds from the Xbox 360 again:
You have to disable the automatic screen detection inside the Xbox 360 console settings. The console will restart and after this restart I was able to hear again.