Picture is dark and audio sounds strange.
The screen can become dim and the sound can go on and off while watching TV.
Things to check
① Do you have the remote control(magic/general) that was given by LG when you purchased the TV?
② Do you see Mute UI(magic) or Mute button(general) on the remote control? (In the middle of remote control)
How to fix
AV might be adjusted so the screen can become dim and the sound might go on and off.
In a simple way, you can reset AV, then it will be initialized as if it is fresh from factory.
(Applied to LM6000 and higher_L9/MKT_S,W Ver:4.04.05 and higher)
① Push Mute button of magic/general remote control three consecutive times.
② If “Yes/No” window pops up, select yes by pushing OK button of remote control.
③ Audio/Video will be initialized.
(ex. Audio/Video: Standard, Power Saving Mode: Off) etc..