new computer i love it but i have no sound HELP PLEASE

ok so i bought a new computer and i love it. It is a

CyberpowerPC Gamer Ultra 5212 Athlon 64 X2 5200+ 2GB DDR2 250GB NVIDIA GeForce 8400 GS Windows Vista Home Premium - Retail. I bought it off of newegg.

Motherboard is Chipset NVIDIA nForce 560 SLI Chipset
Motherboard Name ASUS M2N SLI

It has a 7.1 sound built into the mother board but the problems is i get no sound out of the output in the back of the computer. I have a headphone jack on top of the computer and i get great quality sound out of it. But I am stuck, when i plug into the jack on the back of the computer i have to turn my sound way up and I get a fait sound of my music( i think it might be something with volgate i was reading in another post that had a simillar problem). This happened to some other people on new egg on the reviews they said that they fixed it but they don’t say how to. Does any one got any ideas???

Thanks