Have you sorted this yet? If not, the best connection would be by using an Optical cable, to go from the PS3 to the LG Home Cinema amp. Looking at the spec of the amp though, it only has 1 HDMI input? Is that it?
Edit: Forgot to add, if you do have an optical input on the amp, you will have to go into the settings and change the sound output to optical from HDMI. I think this is actually in the video settings, rather than sound settings!