Upmix line-in from stereo to 5.1

I have an audio device that goes to my PC via line-in but I only get the two front speakers to give sound. I need a means of upmixing/cloning the sound to the rear speakers. Line-in is usually just a pass-through on the soundcard. I need this to work without having any specific music player loaded.

Sound card in an SBLive 5.1 and I have the LiveDrv-Uni pack installed.

Line-in is stereo, whereas SoundBlaster Live! 5.1s push two channels per port, across three ports (front, rear, center/sub).  You'll be looking for a software solution.

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I know that already. What I mean is I can't use something like Creative PlayCenter which has CMSS. I want something thats always on.