For 5.1 sound you will either have to run the HDMI through the home cinema, or as said get an optical lead and split the sound off through the adapter as shown somewhere in this link (assuming you don't have the core pack, which doesn't supply said adapter).
http://www.avforums.com/forums/showthread.php?t=236980#postmenu_1889579However....what do you mean by 'home cinema system'. Do you mean a separate amp, or a DVD player with 5.1 sound or what?
Some DVD player with built in 5.1 sound don't allow you to connect in a external source to get 5.1, you are stuck with just DVDs outputting 5.1 - I came across one last weekend that a friend had bought.