That appears to have 5.1 sound. Have you connected all three leads from the speaker system to the PC properlY?
Go to the Control Panel/Hardware and Sound and finally the Sound Panel. In the playback section, your "speakers" should be set as the default audio playback device. Click on the speaker Icon to highlight it and then click the "Configure" button. Make sure you have 5.1 set as the speaker type. You can test the speakers there to see if they all sound properly. If not, there may be a problem with your speaker system and not the PC.