If the speaker inside the projeector is worthless, then you will need some sort of amplifier/volume control to get audio into the room. Realistically, a cheap A/V receiver can often do this at a far more cost effective level than a stand alone amplifier/volume control setup.
A 7.1 A/V receiver with speakers throughout the room running STEREO, not surround mode, so that all the speakers get equal volume works very well and can be had for a very reasonable price. It depends on the room, how many are listening, how permanent the setup is, etc., etc. It is the main reason why there are tons of commercial A/V installers in this world - they can do it for you easily and get it right - then give you a warranty on things.
