I honestly can't say what it could be since it happens with the audio going straight from the stb to the tv.
It could be the signal coming from the ont. I dont think it could be the stb itself. Maybe someone else will help on here.
Have you tried using component cables instead of the hdmi or maybe a different hdmi cable?
Have you tried using the in home agent to reset everything?
Might be worth calling Verizon and see if they can do some trouble shooting and maybe send a tech out if they can't find a solution.
Is this is all channels, all the time? If so, sorry I have no ideas; but
if it intermittent and only one channel at time, then it may just be that the voice and picture recordings are out of sync. When you watch live TV, your DVR is recording it in the background (This enables you to use the backup feature or record the whole show by pressing REC anytime during the show.) When to go to another channel and then go back to the original, sometimes the picture and voice get out of sync.
Try hitting the Prev button, then the Next button on the remote to go back and forward until they sync up. I have my Prev set to go back 30 seconds and my Next button to go forward 10 seconds. So if I go back once, then I can go forward up to three times to get back to where I was, and this always gets the picture and voice back in sync for me.