So, I have a 50” Vizio flat screen with a sound bar. I lost volume about 2 months ago on the TV but not the sound bar. I unplugged my sound bar and started trying different things.
By coincidence my 8 year old said just turn up the volume dad, which he did! My tv never lost volume, you just had to turn it up to 60 before you could hear it. The sound was better after I went into settings and turned on volume leveling, but still requires at least 30 or more on the volume before you can hear it. I would recommend cranking your volume all the way up and trying again. None of this started until Vizio updated my tv.
hope this helps