Shortly after Enslaved's launch we were informed that the game was only outputting stereo for those using optical audio output for surround sound on PS3. We were told by Namco today that it is currently working on a title update to support 5.1 sound output.

A Namco rep tells Joystiq, "[The update] will be ready around the same time as the paid DLC, which will be available within the next couple of months. Details on that DLC will be announced soon."

So, good news: patch incoming. Bad news: gonna take a while to fix. The game does currently support 5.1 surround via HDMI (provided you have the appropriate receiver).
Source- Joystiq