Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
Your Home Cinema does have ARC capability, but there are several settings on the unit that have to be set --
1 Radio EXT IN button has to select the ARC input on the home theater
2 Video Priority has to be turned on in the Home Theater's menu
3 Video Format has to be set to HDMI on the unit too.
4 HDMI AV out on your Home Cinema has to be connected to the TV's HDMI In, and it's usually labled as the ARC input on the TV set.
We really could use your TV's model number to verify its setup 100%, but usually you do have to turn the TV's speakers off in the Audio menu and select RECIEVER as the sound source.
If your TV has a choice between Stereo/surround, most TV channels are Stereo, not Surround. You may find a choice between PCM stereo and Bitstream Audio (surround).
I don't know if there may be a choice between ARC and OPT out in the LG, but it may be a possibility.
The tuner and any equipment connected to the TV set with HDMI cables should transfer Audio out of the TV's HDMI, to the Home Theater's HDMI.
Scart, or any of the RCA connected inputs are not guaranteed to work with HDMI since those are Analogue audio, not Digital Audio.
If we have your model number on the TV, and if we know what connections you are connected to on the TV set will help us to figure out why you aren't getting sound out of the TV to the Home Theater (HT) unit.
So reply back to me about those TV related questions, and we'll get started, ok?