Audio Out options on Unibody?

What you need is a mini toslink (mbp) to toslink (reciever, like logitech z-5500)

Here it is from Monoprice :
http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10229&cs_id=1022902&p_id=1557&seq=1&format=2#description

I ordered one about three years ago from them as well as a variety of other cables and the quality/price is unbeatable. If you browse around the site there are a variety of lengths available.

Unfortunately if 5.1 is your goal you won't be able to run this from your macbook pro out of the box.

This would be the back of the subwoofer for the product you ordered :



On the left side are the three analog inputs, which would be fine if you had a 5.1 channel card, but because the mbp only supports analog stereo, and this system doesn't have optical inputs, you'll only be able to utilize these for stereo.

Your other option, while I would suggest springing for a higher end surround system, would be to get something like the Creative X-Fi usb card, which will give you the necissary outputs. I don't know if it works on OS X , however.