connecting a lg surround sound system?

With regards to an LG surround system model HT503PH, I have wired up the speakers and then tried to check if they were working using a DVD. All speakers were working but the two tall speakers at the rear were not producing a great deal of sound. Then when I tried using the system with cable/virgin media there was no sound produced from any speakers at all. I was wondering if you could provide an insight into why this is happening.