jp9000/OBS — pull request #59 by hwdro (2013-01-11 14:08:07+00:00)
Title: Replace existing 5.1 downmix code

It's not perfect but it should do it. For more details read the comments inside the patch.
Alternative, maybe,
L = FL * 0.3694 +C * 0.2612 + RL * 0.3694
R = FR * 0.3694 +C * 0.2612 + RR * 0.3694