What receiver?
You should be able to use optical for audio. You can use the video part of the multimedia cable and let the audio part dangle then tell the PS3 to send audio over an
optical cable. You'll be able to get 5.1 to the receiver that way.
In any case, with just a stereo connection you have to rely on your receiver to matrix sound to your sub, center, and rear speakers. It's possible you have it on a setting that ignores your rear speakers. Should be a button on the front that lets you choose from different stereo and surround modes.
You could also have your wires crossed on your rear speakers, but I assume they were working before and you didn't change those when you set up your ps3.