It's been discussed so I'll keep it short. Because of HDCP rules, if a TV has speakers then any passthrough will be stereo with a few exceptions. Over The Air and Apps are one but for DVD you need to send the audio directly to the surround sound system.
Any decent retailer should be able to tell you more.
I am connecting my sky box to HDMI 1 as well as the optical cable. I only get sound from my front speakers not full surround sound. I do get full surround when playing a DVD. Should I be able to get full surround sound from my external devices