I hear this a lot after my productions. It's happened for years.
2004 (Yes, that really was the year): "I could hear your cast!" (and that was a real quote from a parent who sounded relieved that projection actually happened).
2009 (that is an approximation): "We watched you rehearse outside and wondered why you were making them brave the heat but when your show went on, we could hear them!" (from the costumers)
I'm amazed that they all sound so amazed. It finally dawned on me that perhaps they are so pleased because it doesn't happen as often as it should.
Projection is basic. If an audience cannot hear the performer, no matter how wonderful the acting, there's no reason to have an audience. None. If the person in the back row cannot hear the performers, why come? If it happens often with your group, audience members may stop coming. I did, until the group finally got a theatre with better acoustics. Even so, I still try to sit up front.
Directors, please be vigilant during the rehearsal process and get your cast to project. Performers, more often you aren't as loud as you think--be louder.
Do not rehearse in small spaces if your performance space is huge.
Rehearse outside if you'll be performing outdoors.
Because sound bounces off walls and in small spaces performers sound loud. They think they're loud enough. They practice being loud for that space and when you switch over, they will be as loud as they were in the small room, which won't cut it for the larger venue. If you must rehearse in a small room, then rattle the windows with your voice. Don't let the performers sound normal. Eventually, as the director, you need to sit in the back row and listen, allowing yourself to let go of the script you know and try to hear and understand the lines from an audience's viewpoint, an audience who hasn't heard the script 100 times yet.
If you are a speaker and you have a choice of using a microphone or projecting, use the microphone unless you KNOW that you can project to the back of the room. And when I saw "you KNOW" I mean you've rehearsed with someone in the back row listening and can verify you can be heard.
If you're directing/producing a play, do not depend on microphones. The moment your cast thinks there will be microphones hanging down from the ceiling, they get too quiet for the mics to pick them up. Or if individuals will be mic-ed, then the show sounds unbalanced because those not mic-ed don't seem loud enough by comparison. Besides, I've never met a mic that doesn't go through technical difficulties. Forget it. Get the cast to project, unless you're on Broadway or in DisneyLand/World.
Projection cannot be neglected. To reiterate, if the cast can't be heard, why bother coming? So get back to the basics and project. Your audiences, present and future, will appreciate it.