I got the DTT3500 set so i might be bale to help you, how to do hook up the decoder to the computer? what type of connaction? (i dont know anything about that sound card of yours) how many speakers dose it support?
did you go into the configuration panel for the sound driver? IF your sound card supports multiple speakers then you should have multiple speaker playback..you should be able to run a sound check from within the driver or from the decoder..hope that helps.