The hdmi on your home theatre system is output only, ie to tv. It would appear there are no inputs other than rca phono (red/white for right/left), so no 5.1 from your ps3 i'm afraid.
Best you'll get is 2ch stereo if you connect the tv's audio out to the home theatre system. There may be a dolby pro logic ii option, which'll make the 2ch stereo into a "virtual" surround effect.
As others have said to get the best from your ps3 an upgrade is required really!
