Hi. I'm Steve. I'm sorry to hear about your TV. I will try my best to give you a precise and accurate answer.
You get sound when you play a DVD because the DVD player is built in to the LG HTS.
To get audio from your cable service you have to actually send the audio to the HTS.
You can either connect the TV's audio output to the HTS or the cable box's audio output to the HTS. Typically, the TV's output will be a digital one, so you need a corresponding digital input on the rear of the HTS.
The cable box will also have a digital as well as a regular pair of red/white RCA jacks. You can use those.
That's an overview. Tell me if you need more info.
Just ask if you need more help on this issue. I will be here.
Thanks very much.
I'm sorry. HTS is the Home Theater System - the LG.