Just go into 'sound' in system preferences and select Airplay-airport express. That is, assuming that you are using Lion. They did away with Airtunes a while ago and now it's called 'Airplay'. Airplay is used across all Apple devices including iOS iPads and iPhones.

If you are using older technology, then I haven't a clue.

note: the new Airport Express's (I haven't tested previous versions) latency is good enough to use for movies so it should work fine.