No sound when using flex

Good morning, 

We plugged our Comcast Flex in and it works fine, however we get no sound when we use it.  When I go back to Netflix, Hulu, Etc the sound works fine. We have a Vizio Surround Sound system and I cannot figure out why the sound isnt working. I went through the settings in both the TV and the Flex.  Currently I have the HDMI cord plugged into the TV, and the power cord plugged in. Then I have my optical cord for the Surround Sound plugged into the TV, like normal. I appreciate any help