Sure, no problem! Hopefully someone comes along and provides a bit more of a concrete answer when dealing with your sound system, but I know that for me and my TV (with a sound system as well) that the audio works fine.

EDIT: Actually, my mistake! I overlooked the fact that a 2009 model for the MBP does not support audio out through the MiniDisplay port. You'll need something along these lines instead (~$50). Or you can read through this Apple Community Support thread, as it discusses similar desires of hooking up an MBP with audio.