No sound on HD channel 7 HD

"""""""I just bought a VIZIO VF551XVT 55" HD TV . I hooked it to a Motorola Cable box using an HDMI cable-provider is COX cable.

I too am having the same issues. If I go from a standard def to high def channel I lose the sound. If I flip to another HD channel sometimes the sound comes back sometimes it does not. Even if it does come back it's not back for all HD channels. UNLESS I turn the TV on/off OR switch modes on the TV in some way (one way is to push he "media" button on the TV remote).""""""""

I having same problem with same tv. but only on one channel. Cable connected from the wall directly to TV. I've spent an hour today between Vizio and Comcast tech. support.
My solution to that is a "MTS" button on the remote control. You Have to push it twice. This sets up sound to "English 5.1 HD". I'm guessing this info is programed and transmitted in the video and audio digital signal. For some reason it set wrong way at the source.
Try it. I hope it helps.