PS3 5.1 Audio problem

[McGarnigal]

I've been using my ps3 as a blu/dvd player with all the formats my receiver can support for years. I just downloaded an HD episode of Lost from the psn store. The description said 5.1, so when It started playing it defaulted to stereo, but I hit the audio option button. It said English 2, but there is no sound. I hit select and it said i was some form of compressed AAC 5.1, i don't see any audio options on the ps3 for additional compressed 5.1 formats, just larger pcm stereo options..