Another favourite with this set is after you've been using your i.pod etc headphones via 6.5mm adaptor,you remove the headphones,but leave the adaptor still in the socket.This cuts out the speakers of course!
If not the above,do you get sound via peripheral devices eg,vcr,sat etc?connected to the av sockets?
You may have a problem with the supply to the amplifier stage,or the muting circuit with mutes the switch on noises etc.
These sets are pretty much "bombproof",so I would'nt expect the problem to be major.
Where abouts are you?