Vizio 42" 1080p HDTV LCD.

If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/

Sounds like tuner fault.

I have a similar problem with my new 37 inch vizio. Watching on cable and the sounds gets lower and lower until i can hardly hear it. Must switch channels back and forth to restore the sound level of a particular channel. Does anybody have a solution ?

take the TV to the nearest Vizio dealer to get the screen replaced...but be warned it would be expensive

It sound like you may have sound processor problem which is part of the main board, I would also look at the 12vdc from power supply to make sure it is clean DC.
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http://en.wikipedia.org/wiki/Capacitor_plague

Digital converter box have to be replaced.

My 37 inch vizio did the same thing. go to your menu screen on the tv and go sound. My tv has an option to turn speakers on/off. Once I turned my speakers off and the back on again everything worked great. hope I helped! if I did please let me know! youtube.com/xniax1

our brand new Vizio 32 HDTV has poor sound quality, so irritating in it's flat muffled character that I can't watch it. My electrical engineer husband as attempted to change it (he does not hear the difference between the Vizio and our two Sony...there is a BIG difference) by turning off the surround sound. But to me nothing has changed. What do you think? it sounds a little bit like an old radio or a broadcast from inside a closet. Hmmm

My 32" vizio I bought a week ago does the same things.

Buy an external sound system for the tv. Turn the speakers off in the TV menu. Hook the sound system up to the out speaker hookups on the tv.

You will not only have better found, you will fix the speaker buzz problem.