unable to hear sound when center channel input is turned on for surround sound option

I have a HD-52G786 tv. I am trying to utilize the tv as my center channel speaker when watching a DVD. When I switch the option to Center Channel ON, it is muted. the book states "If there is no video signal present, the TV will be muted." I hook up the DVD to my tv via HDMI cable and a digital optic from my amp to the TV. Is there an additional connection that I need to make? I assume the HDMI cable provides the video signal from the DVD. any help is appreciated.

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