So often you can tell a person is dumb because you hear them long before you see them. Why the common correlation?
Never heard of this, nor encountered it. Do you have a cite to back up your premise?
Assuming it’s true, it might be because they haven’t mastered the idea of “social context” and don’t know how to pick up on the clues everyone else relies on to know how to speak appropriately. Being unintelligent isn’t about lacking specific knowledge, it’s about being unable to piece together the knowledge you have. (People like to phrase this in terms of being “slow”, but at some point, it really is more about “able” vs “unable”. I will never compose a symphony the equal of Mozart no matter how much time I had, and that’s that.)
You are confusing social ineptitude with intelligence. It’s very common for highly intelligent people to be socially inept, and equally common for “unintelligent” people to be very socially facile and easy-going.
It’s quite possible that you are perceiving them as dumb because they are loud, rather than them being loud because they are dumb. Plus, if someone is dumb and quiet how would you know they are dumb at all?
Hearing a dumb person? Isn’t that an oxymoron?
Oh, you meant stupid… 
In a related vein, one of Jackmannii’s Rules of Dining Out states that, of the people at an adjoining table(s), the one with the most annoying voice talks the most.
i HAVE NO IDEA WHAT YOU’RE TALKING ABOUT!!
Maybe they think, because everyone ignores them, that they need to speak louder?
(it isn’t we didn’t hear you, it is because we think what you said is not worthy of response - a lesson they cannot comprehend.)
Some dumb people learn to keep quiet during intelligent conversations above there comprehension. Too often, these are school kids in classrooms with an overworked, underpaid teacher who is quite happy not to have to correct yet another student who doesn’t quite get it.
And so the dumb kid falls further and further behind - and thinks the job cleaning up at $10/hr is a better deal than yet more D and pity C’s.
For the same reason that the loudest, dumbest, most Fox-news-spewing person ALWAYS sits next to my table in a restaurant or is next to me on my 12-hour flight.
Because they’re deaf?
Maybe they drown out new information. They started out just loud, but years of it has made them the dummies they are today.
Entertainment?
They’re not necessarily dumb - in fact, some may be scarily smart - but many people I have encountered on the autistic spectrum seem to not be able to modulate their voices appropriately. I don’t know if that’ part of the effect you are seeing.
Individuals with Down Syndrome / trisomy 21 are loud because they have poor physical control of their voice: it’s part of the syndrome.
Not that I think that’s what you meant, I just thought I’d add it.
I would say it has more to do with insecurity than outright lack of intelligence.
No mention of Confirmation Bias yet?
It is likely that Stringbean has encountered many “dumb” people who are not loud, but does not notice a correlation for dumb = not loud.
Do you notice a correlation for intelligent = voice at soft-to-moderate levels?
Actually, I’m proposing a lack of intelligence as a possible cause for social ineptitude, which it certainly can be.
Ahem:
Perhaps I should have posted more loudly. 