OK, so you have a stereo input and you want a set of 5.1 speakers. So you need to decide how you are going to expand the input (2 channels) into the output (6 channels). Three of these are fairly easy, your left and right go to front left and right, whatever the sub threshold is will go to the sub (e.g. below 20KHz).
What would you like to send to the other 3 channels? Generally you would not listen to music in surround sound, as it is designed for stereo.
Dolby require a license for their technology, which is arguably the most widely used, because they invented it and patented it. This will also apply to Dolby Pro Logic II, which encodes a surround sound signal in a stereo stream.