You can get a cable at Radioshack that does from a digital plug (plugs into speaker port of all sound cards) to 2 RCA jacks. This will allow you to play your system through your stereo speakers, provided you have RCA hookups on the back of your stereo.
As for using both computer speakers and the above setup, you can get "Y" adapters to hook up 2 sets of speakers. With this, not all sound cards like this and I would suggest keeping the packaging for the wires for returns if it does not work. I have had one system that this worked on and one system that it did not work on. I could never figure out why it did not work on the second system.
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