Sorry for trying to be helpful. ;\) As indicated, I don't claim to have the best solution, just a working solution.

I'll warily add in the possibility that you check the PS3's setting where where 5.1 is turned on/off for the optical port.

I can't recall the exact menu location but I know that you can independently switch the optical port on your PS3 from stereo to 5.1 and if hard set to stereo, you won't get 5.1 out that port.

With great power comes Awesome irresponsibility.