There is no need for a headphone jack on the sound system for this to work.
Im guessing that you have RCA Audio inputs on the back of your reciever correct? If so, then you just need to buy a part at radioshack for a few bucks that converts a standard computer audio plug to RCA plugs, then find some cables, connect it to the RCA conenctor on the computer, and connect it to the Audio In on the back of your reciever.