5.1 systems have 3 x 3,5 jack output(orange, green and black). In order to have true surround sound you need to have a 5.1+ capable motherboard (they usually have 6 jack holes - blue, pink, orange, black green, grey) or a dedicated sound card and you need to plug in all the jacks.
Pink = Microphone In (mono)
Blue = Line-In (stereo)
Green = Line-Out / Front Speakers / Headphones
Orange = Center Speaker and Subwoofer
Black = Rear Surround Speakers (5.1, 7.1 systems)
Grey = Mid Surround Speakers (7.1 systems)
So make sure you connect all of them to get 5.1 true sound.