From the little I know about home cinema it seems to me that you are not linking the sound from the TV into the theatre box.
If you are watching TV via an aerial then you need to get the sound OUT of the TV into the Theatre box. The TV should have some audio out connectors.
If you are watching TV via Sky or Virgin you need to connect the audio out connectors on the Sky or Virgin box to the theatre box.
I had a Audio Visual (AV) receiver a few years ago, and connecting it up was a nightmare, there seemed to be wires going everywhere.