You surorund sound receiver has an optical input. Buy an optical cable and use it from the PS3 to your receiver and change the PS3 audio to go out from it instead of the HDMI.
The optical is also called an SPDIF. You should be able to get an optical cable online for $5. If you go to best buy or somehwere it will probably be $15 or so. Quality with them doesn't matter at all. There would be no difference between a $5 or $40 one unless you are doing a really long run with it.
Here is a link to one
http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10229&cs_id=1022904&p_id=2765&seq=1&format=2
It drives me crazy when people setup a surround sound and just use the red and white RCA cables connected to a surround sound receiver. They will not give you surround sound, you have to use the right kind of cables to get it. I went to a friends house not to long ago and he had a decent $1200 or so surround sound setup and was just using it in pro logic because he didn't realize how you were suppose to hook it up. I took him a cable and it sounded much better.