Surround Sound only 2 speakers?

Hi, I have a Panasonic home theater system (SA-HT330) which gives out 5.1 surround sound via white and red coaxial cables. I also have a Samsung 50" series 4 plasma tv with SRS TS XT ( used to out 5.1 surround sound). I have a Dreambox connected to my tv with Sky HD channels. The theater system is connected to my TV and I can play DVD's on the theater system in full surround sound so the speakers are working fine, however when I change my theater system to TV mode and play the sound from my Dreambox it only plays through 2 speakers. I have changed a AC3 downmix setting on the dreambox to nowhich is supposed to allow surround sound, and have turned on SRS TS XT on my TV but I can still only get it through 2 speakers, any help would be greatly appreciated, thanks..