This kind of falls into my field so I'll help out
If it only has the two red and white RCA jacks in the back, unfortunately there is no way to get true 5.1 surround sound out of the system no matter what sound card or adapters you get. The red and white jacks carry the right and left channels of audio respectively leaving no room for center, surround left, surround right, and the LFE (subwoofer) tracks.
The only way to get true 5.1 soundtracks is by using one of 4 connections:
1)Toslink/Optical Digital - this is a fiber optic cable
2)Coaxial Digital - this looks like a single RCA jack but it's gold colored
3)HDMI - we all know what this looks like
4)5.1 analogue in - a set of 6 RCA jacks (one for each speaker in a 5.1 system)
Now, you may be asking "then why did it include 5 speakers and a sub?" That is because they can "simulate" a surround sound experience using only the left and right channels through matrixing. I won't go into too much detail but it can essentially detect the levels of phase between the left and right speakers and re appropriate different sounds (i.e. dialogue) to the center or surrounds. Dolby Pro Logic is an example of this.
It used to sound pretty terrible compared to true 5.1 experiences but over time with technology getting better, these matrixing algorithms have become so good recently that it's hard to tell the simulated tracks from the true 5.1 tracks in a lot of cases unless you have really good ears. That being said, true 5.1 tracks will still deliver the experience that the sound engineers in the mixing room actually created.
What you can do to improve your listening experience? Look for audio devices that will output sound at up to 24bit and 192kHz sample rates (make sure to find the audio files that are recorded in 192kHz). That will deliver much greater clarity and depth if you have good ears. Be warned that it's difficult to find audio files in these sample rates though. They're just not circulated very much.
