There are some menu settings on the PS3 that control what it does with the sound. I think the manual says that it defaults to audio output from all connectors, but I had problems getting sound working when I set mine up.
You can choose which output to direct the sound to: HDMI, AV-multi or Optical. I guess that it might not be set up correctly for your situation.
Go to Settings -> Sound Settings -> Audio Output settings.
For instance: you might use HDMI for video to the projector, but optical for audio to your home cinema amp.
Have a tweak with the options. You might want to experiment with the AV-multi connector to get basic 2-ch audio to the amp - if you can get that working, then take it from there.