Hi ...I'm new to this home cinema so apologies if i come across as a technical bozo

I have just bought an LG home cinema blueray system HB405SU.Although it says it has Dolby true hd , digital plus , and dts hd there seems to be nothing in the instruction book on how to tell what system is playing back .I'm watching a standard dvd of "There will be blood" which is in dolby digital 5.1 but hardly anything seems to be coming from the back speakers I have done the sound tests and all seems to be fine.

I have the system connected to my standard definition samsung plasma via HDMI and use the tv's optical out into the LG's optical in.

Also can anyone advise on what resolution I set the hdmi to ?

many thanks

You shouldn't need to have optical out from TV into LG unless you have other sources of content e.g. Freeview via the TV.

If barely anything is coming out, it means something is coming out meaning they are working. The DVD might not have a lot to output through rear speakers. Perhaps ask for recommendations of a good "test movie".

The Normandy landing scene in Saving Private Ryan will give a 5.1 system a workout, alternatively the attack scene in Pearl Harbor (their harbour their spelling )

TVs don't pass on 5.1 from external sources onto their optical output. They only do this for their internal tuner. It might be that the LG is using the TV's optical output in preference to the sound from the DVD? Try removing the optical connection and see what happens.

I suspect the TV's optical out will carry only a downmixed stereo signal. I suggest you use an optical feed directly into the amp. But if it can do HD audio formats, won't it have HDMI inputs and outputs?

ETA: beaten to it, and it's an all-in-one system. Do what the previous poster suggested!