This is my exact problem. It starts at around 9:30-10:00 at night. Sound just goes in and out. I have had a technician out and replaced the boxes. I have been calling Verizon and they keep giving me the run around. We are very close to dropping them. This last update destroyed my TV watching. I know that this is not an internal problem but they keep acting like it is. They also told me if they replace any cables they will charge me. I have had it!!!
Try going into service menu, power off at box, ok 2x, left arrow, choose user menu, additional HDMI settings, change audio to LPCM, ok, power off. That might work for you.