Speakers not supporting

I recently bought a Phillips 5.1 speaker system (MMS 5.500i), and i've Intel 845 GVSR mobo. ; it has integrated sound card Realtek AC 97 Audio; The problem is the 5.1 system is not working when it is connected to the PC...
I tried to change the Line in func. but in Vain.. So any ideas how to make tghis system work???