Use two 4.7k resistors connected from the left and right channels to a common point to generate a new Mono signal. This can be used for the center channel since the center information and LFE is mixed in this channels by the player software. Sub can connected on this signal too if a lowpass filter is present. Otherwise use a simple RC lowpass to generate the Sub - Channel from this new Mono source. You can connect your 4.0 out from Soundcard to 5.1 input on a Surround Amp ;)

Don't beat me, it looks like some new 5.1 sound cards with 4 channel YM 744 soundchip are doing it this way (buffered with OPV)