Not sure you need to do that. My projector has small speakers in it that can be heard pretty well up to about 15 feet away when cranked all the way up. Good enough for a small group of people at my site. When it's time for "Monday night at the movies" I take the projector down to the pavillion and feed the audio through a 100 watt portable system. The movies always end before quiet time and anybody in the park is welcome to come watch and make selection suggestions. Has gotten to be fairly popular among the guests.
The point is, the small 1 or 2 watt speakers built into the projector don't carry very far and you can have a few friends over to watch without disturbing your neighbors or spending a fortune on audio gear.