Using the sound from your projector is like putting together a full-fledged sound system for your home theater--separate components,, monoblocks, etc--together with an old black and white 13 inch surveillance monitor to watch your movies on. It just doesn't make sense, the only exception being if you're having an outdoor screening, and even then I would use a boombox or something like that as Bohanna already suggested.
What are you using to feed an image to the projector? What kind of audio outputs does it have? Tell us that and we'll go from there.