it would be a bit dodge but if you turned it down low enough you might be able to get away with using the headphone socket... wouldn't risk it with an expensive PC/hi-fi and be careful of the levels if you did that!
or you could run the headphone out through your mixer and use the trim to bring it down to line levels? that would depend on whether your mixer could handle it though, again be very careful!!
i got a wire that cost about £3 out of dixons, it has a headfone jack at 1 end and 2 aux connectors at the other, connected the 2 aux bits to the speaker outputs and the headfone end to the mic socket on my pc...used wavepad which u can get for free to record it and bang
i think theres an alternative way of connecting the stereo to the pc with the same wire but i couldnt remember it so that way worked