You should be able to use a 3.5mm mini stereo connector to 2 RCA to connect the inputs or outputs of your sound card to your audio system.
If your sound card has digital audio SPDIF in or out and your audio system has SPDIF coax in or out you should be able to use a special cable to hook them up. This will depend on your sound card because some use a proprietery connector while others use a digital coax in/out.
If you can post your system specs and the model numbers of your sound card and audio system we can figure out what you have to work with.
Semper fi carry on