Help with a cheap HTIB

[Pojomofo]

So I recently purchased a cheap HTiB (RCA RTD217) for my bedroom. It has a DVD player built-in, and the surround sound sounds great when playing a DVD, but only stero with TV. This unit only has a red/white Audio in, but has Dolby Prologic II, which I thought was supposed to turn it into 5.1, but isnt.

Am I expecting it to do something its not capable of? Any tips would help. Thanks