the way a computer soundcard outputs on the 3.5mm jacks is different than how a receiver expects to receive it.
my first choice would be to use hdmi IF your video card supports sound out via hdmi and if your receiver has hdmi inputs.
my second choice would be to use spdif optical IF your receiver supports spdif input.
if you try and convert the 3.5mm pc speaker connections to use on a receiver its likely to be frustrating as pc speakers are set up a little differently than most home theatre systems i've seen. i'm sure there is a conversion box to make this easy but this would be my last choice.