To put it simply, if you dont have anything plugged into "any" of the TV's external sockets and you have also made doubly sure that the sound hasnt been muted on the remote control, then there really isn't anything you can do as its obvious that an internal fault has developed. This comment made along the lines that its assumed you have switched the set off "completely" (not just standby) and then re-powered it and the situation hasnt changed.