If the customer is using some type of converter box, I would feed the audio outputs to an amp and bypass the TV audio totally. That way, the TV would not have to be on to listen to audio.
I know of no TV (including the PT-43LC14) that will operate without the lamp being on. Even using a defective lamp will not pass the startup sequence and have the TV operational.
That which may be known of God is evident within man, for God has shown it to them, so that they are without excuse. (Romans 1:19-20)