the only way is if your soundcard has a digital output and there is a matching digital input on the DVD player.
That way the raw signal from the content on the pc will be decoded by the DVD player and should then go through the 5.1 surround speakers (assuming the source is 5.1 encoded)
Other than that you are stuck with stereo with your present set-up.
Is the soundcard 5.1 compatible? If so could you not just plug the speakers directly into the pc?