I only hear sound out of two speakers........

Hi, i have the EV sonicXS 4.1 speaker system and only two speakers work at a time. When i plug the line level input cable from the subwoofer into the back of my computer (the headphone jack audio out) the two corresponding ports on the subwoofer which i plug any two speakers in work, but the other two do not. I have tried messing with all the plugs and moving them around but it seems the best sound i get is only out of two speakers. I have an Hp pavilion and when i plug the other line level input cables into the front headphone audio out jack at the same time it takes over and plays from the two corresponding speakers for that one. I dont know what to do! please help. thanks
-daniel