Have you tried the Alsa sound driver? I've just checked if the Surround L/R audio Connnector works with Alsa 0.90rc8 (Asus A7N8X mother board) . Seems ok, so if you have the mixer setting "Duplicate Front" checked you can test this analog output with regular stereo source (xmms, etc). Then use the Surround slider for volume.
But then maybe that's not what you were looking for.