Have you actually set the PS3 to output audio over HDMI?
For the old (Fatboy) Playstation3:
Select Sound Settings
Select HDMI
Select AUTO
Check that all the DD, DTS and all PCM up to 7.1 are ticked.
Save
Go to BD/DVD settings (withing Video I think)
Select audio output format as PCM and save.
For the new slimline Playstation3:
Select Sound Settings
Select HDMI
Select AUTO
Check that all the DD, DTS and all PCM up to 7.1 are ticked.
Save
Go to the BD/DVD settings
Select audio output format as bitstream and save.
Make sure your amp is in auto mode