You don't say what models STB or TV you have, or how they are connected, but this issue is sometimes solved for those using HDMI connections and Visio (and other brands) of TVs (or some receivers) and 7XXX STBs by changing the following in the hidden settings menu of the STB (from a post on DSL Reports). You might give this a try.
Power off the Set top box.
Non the remote hit Select
On the remote immediately hit Menu
You will see a white screen and an all text menu called USER SETTINGS. Use the arrow keys to move around.
Scroll down to a line that says "ADDITIONAL HDMI SETTINGS" and hit a right arrow to get into the next level of the menu
There is a sound setting called AUDIO OUTPUT It says AUTO Scroll to it and change it to L-PCM.
Thanks - I do have the Visio connected with the HDMI cable. I have the upgraded STB but can't get home to see the model. I'll try what you said, then post back if it continues. Thanks a lot