We have had this same problem. Off and on for years, mostly when we were gone for a time and came back. Resetting everything (cable box, tv, etc) seem to work. Then one day the sound went off in the middle of it working. resetting did not work. We were told it was most likely the main board. We found a main board on ebay. Could not find one anywhere else. We replaced the board. Still no tv speaker sound. I told my husband to try to turn the volume up on the tv and not the remote, he looked at me like I was nuts! I read this in one of my many searches. Guess what? The sound came on, and now the remote worked too. Then about 10 minutes in, the sound went off again. Vizio obviously has some kind of glitch and they must be aware of it because there are 100’s of people with this problem. and not just with my model. Vizio does not make 3D tv’s anymore and neither does anyone else, we have over 200 3D movies so we were bound and determined to keep this one working. Well now we have an extra main board. We have model M3D650SV purchased in 2012 . When I try to use the app to watch Netflix or Hulu, no sound, so I watch Netflix and Hulu from the DVD player so I get the sound through the Home theater system. We are able to get sound through our home theater system, so we will just use that. We tried to hook up a sound bar to the Video through RCA Video out, that didn’t work either. The Home Theater system is the only way we get sound now. So if that is an option for you, at least you’ll get sound.