obviously check your cables. You didn't identify what type you were using. HDMI? RCA? etc.
Check your audio settings both in the reqular menus and hidden menus to see if you have something restricted, and to try PCM vs Dolby Digital.
Do you have more than one STB? if so switch which one you are using to see if its a bad box.
Worse come to worse, contact Verizon Support. You may need a new box.