This is actually a problem that actors face on a regular basis. Let's ignore for a moment the beautiful people and the people playing themselves that are generally represented in film and television, once upon a time actors were people on stages, and in some cases these stages were in larger theaters.
And projection and enunciation are important, because when it comes to plays - there's no bored-looking dude holding a boom mic on the end of a long pole stuffed into a flayed Muppet stuck just outside of the shot. It's especially true these days when you consider that people who actually will get into a car, drive into a major city and spend money on actual tickets to watch people actually perform tend to be of the "older" variety and therefore not necessarily as well equipped anymore to hear what you're saying.
Interestingly enough the ancient Greeks didn't say "go see a play", they would say "go hear a play". It's one of the reasons a place where performances were held was called an auditorium, "place where things are heard".
So I remember in my classical dramatic arts education, back when it was called dramatic arts - I received a thorough grounding in the correct use of the diaphragm and stomach muscle use to forcefully project the voice. As a kid in middle school, I was taught to focus my attention on a point about twenty yards away, a back wall of the place where we rehearsed, and simply do a long vowel channeling the oh in "om" like a beam, striking against the wall and reflecting back, concentrating on being able to feel it reflect back to you. This also includes opening the throat and relaxing the mouth so that it can resonate freely and give you that Brian Blessed volume you need to be heard back in the cheap seats.
As a result, when I've been tasked upon to read in church, the guy cuts the mic to the lectern because with the cathedral's design and my innate ability to project my voice, there's no need for amplification to be heard anywhere in the room. A few of the older folks have actually thanked me for volunteering, as they can somehow hear me better than an amplified speaker.
But the second part of it is exercising the mouth. If you ever watch an actor backstage before a performance, they're usually relaxing the jaw, flexing the tongue, contorting the facial muscles, stretching out and pursing in like an "O". They will usually run through a catalog of phrases where either repeated consonance or difficult passage train the mind and the mouth to modulate the power you've just developed.
"Round, and round, the rugged rock, the ragged rascal ran."
"Rubber baby buggy bumpers".
"Unique New York".
"Red leather, yellow leather".
Sometimes it will just be a stream of consonant/vowel combinations "ra ga ka ba na ma re ge ke be ne me" etc. which to the uninitiated sound like a Tibetan chant.
They'll go through a whole host of these because quite often plays are of a literary merit, where using a larger word seems more "theater" than using a basic, in-use word. And given that many people will flock to see older plays, Dickens, Shakespeare, Ibsen - words in those tend to be longer and more cerebral, even if in Shakespeare's case the humor was about the level of a Jerry Springer episode.
The gist is, they're so used to projecting and enunciating that even though it sounds exaggerated and shouty at close range, in the theater it just works, and it's intelligible without any thought, or any conscious effort - allowing the actor do to his or her job.
You don't have to be an actor for any of this to have merit. I dated a girl in my early 20s, as people are wont to do in an area with a multicultural flair - from Vietnam. Her mother was frustrated that her engineering firm kept sending her to take English classes, even though her understanding of English was better than most.
Given that I knew her in a more familial way than her employers, I was able to get to the root of the problem - people from that area tend to speak in a way, similar to their own language, in which the consonants at the ends of words are not spoken, it's just how they end. To Western ears Vietnamese sounds "strangled" and unenunciated, with the bulk of the meaning being in the vowel tones Westerners can't grasp. (It's a different word if it's rising, falling, falling then rising, or even). Her father laughed over many beers trying to get me to say a certain word, and I was constantly trying to get it right and getting it wrong. "No! That singing!" "No, that eggplant!"
I dug into my repertoire of vocal exercises and said to her, go ahead and overexaggerate the end of the word, I mean really, really speaK the enDahZuh of worDZuh as if you're trying to be funny or sarcastic. "Like this?" She responded, and it sounded dead-on like a native English speaker. She was never asked to take a course again, even though she lapsed into her normal speech patterns back at home.
It doesn't hurt to exercise any and every part of you. Write letters, practice cursive. Breathe from your diaphragm. Floss your teeth. And above all, exercise your mouth to let those big words come right out.
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