Thats a good question. On the back of my computer i have a green plug, red plug, and blue plug for audio. I plug my normal speakers into the green. In my realtek sound manager i can change from 2 channel to 4 channel to 5.1 audio which utilises more of those plugs. Try configuring your sound manager.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.