What I want to do is mix the 5.1 sound from my ps3 with audio (stereo) from my computer. Some games will not let you play custom music with the game.

I really don't know where to begin. Maybe there is a hardware product (cheap) that could do it for me, but I was thinking maybe the computer would do it.

Don't know if its possible though - I use dolby/dts passthru for 5.1 because my receiver decodes it (toslink). I think that would mean I would need to decode the dts and mix the stereo tracks in, then back out to dts. This probably would not work out... any ideas?