With AirVLC you can watch videos on your Mac and play the audio through AirPlay, in sync. AirPlay alone causes a delay in the audio, which makes videos unwatchable. AirVLC helps to overcome that by creating a delay. It uses VLC. No need for a sub-par video player. AirVLC merely controls the awesome video player that you already know and love.
Works with local files and Web videos. This means that you can watch any .avi, .mkv, and .mp4 you have lying around, but it also works with YouTube links. Drag a link from your browser onto AirVLC and you're done.
OS X 10.8 Mountain Lion supports streaming to a single AirPlay speaker out of the box. If you have multiple speakers or use Snow Leopard or Lion, AirVLC can use apps like Porthole to enable AirPlay on older systems and even enable you to stream the audio over multiple AirPlay speakers!