Answer is that you can't. I've played with the settings on the LG and attached sky box but I am still not happy with the sound - too thin. A decent AV amp and speaker system works in a lounge but seems a bit OTT anywhere else in the house!
I have a small and cheap Technika with DVD that we use in the kitchen. Its OK for the price but not a patch on more well known brands that we have elsewhere in the house. The picture quality on the LG in the bedroom is particularly impressive although the sound quality is very poor.