With the headphone outputs you have analog 2 channel stereo only. Now that will allow for ProLogic surround only if sent to a ProLogic decoder/amplifier. In order to get 5.1 digital audio you would need a digital output. They make USB devices to provide a digital output like this one: Behringer U Control UCA202 USB Audio Interface
Then you would need a 5.1 amplifier of some sort. A low cost home theater receiver would be one way to go. Or a complete system like this one: Optical Coaxia Aux 5.1 Surround Sound Speakers
I know you have some speakers already, but you may need to buy a complete system since there are very few, if any, separates anymore.