I too was having problems lastnight when watching a Blu-Ray film(firewall).
I set the sound output to LPCM and started watching the film and noticed i got nothing coming out the rear speakers (i have a 7.1 setup).So i played aground with some settings and no matter what i did with it set at LPCM i couldn't get it to have sound coming out the rears correctly.
In the end i had to set the PS3 to bitstream and use

EX which gave me a decent sound. which to me seemed ridiculess on a Blu-Ray.
There wasnt any sound settings on the disc either and when i put the sound display up it said 5.1in LPCM mode or multichannel in bitstream mode.
To be honest i have not noticed it before i was watching Pirates of carabian 3 other day which sounds awesome.(didn't notice if it had 7.1 soundtrack)
also when i have been watching normal DVD's i have never had to change to DD ex to get full 7.1 it just did it when in DD and sounded perfect.
Anybody know the reason for this Was this basically down to a crap soundtrack on a Blu-Ray movie

or have i got something wrong ?im using the settings as advised by ROM in the first post(thanks by the way).
what listening mode do you(collective you) use to when watching a Blu-Ray with 7.1 speakers and what do you use when watching sd DVD with 7.1?
Sorry to go on a bit but i was getting so angry

last night messing with the settings i nearly turned the film off and went to bed with the huff