Almost ALL TV output is stereo due to HDCP rules. There is a FINE EXCEPTION for Over The Air (OTA) which can be 5.1 but the HDMI/HDCP rule is thus:
If audio or video can be rendered at a device, the content that is passed on must be stereo or standard definition.
Your issue is usually how folk learn about how consumer unfriendly HDMI/HDCP is.
I have a 42LGX TV and just bought a Samsung HT-Z420 5.1 surround used. I set everything up and the surround sound works from the i-pod dock but when audio is coming through the tv it only uses the front two speakers and sub. I have the tv plugged into the surround sound box through an optical cable and the TV specs say that it has an audio decoder so I am not quite sure why it will not work. My only inputs into the TV are a PS3 and HTPC. I have also tried playing audio from the USB and none work with all speakers. Any help would be greatly appreciated.