More specifically, why is there any vocalization at all? When I breathe heavily while working out I don’t vocalize. When I cough I don’t vocalize. What is it about a sneeze that activates the vocal cords?
Note: prior threads asked why we say “achoo” instead of some other sound; I am asking why we make a sound at all.
WAG:
At the start of a sneeze you have a sharp intake of breath. This makes the “AH” sound.
Then the air is expelled and, for some reason, we close our mouth (mostly) on the exhale (thinking about it now I cannot imagine sneezing with my mouth wide open). That action makes a “CHOO” sound.
My father would swear when he sneezed…“AH…Shit” so…¯_(ツ)_/¯
Sneezes involve reflex spasmic forcing of air out of the lungs and through the vocal cords which are also affected by the spasm. That’s gonna make a sound regardless. One can modulate the shape of the cords a bit as it’s happening but the end result is usually a vocalization that can be stereotyped into a few typical categories of sound.
A sneeze is a comparatively violent respiratory action. Such sudden and massive air movements within the respiratory tract are unavoidably going to make a lot of noise even if you leave the vocal cords out of it.
It’s like why a balloon popping is noisy.
Why doesn’t the same thing happen with a cough? It makes a sound but the vocal cords usually don’t get involved.
Coughs are more organized, semi-voluntary responses of muscles. Even so, coughs tend to make noise also. The sound can just be stifled/modulated a bit easier.
Note to Dopers: do not question the wisdom of Dr. Qadgop. Nothing to gain by that.
Thanks for the vote of confidence, but I’ve been wrong before, and will be again. Sometimes I discover my ‘knowledge’ is outdated. Or was wrong in the first place. Live and learn. Which beats live and don’t learn.
Ah-choo seems one of the better onomatopoeia words in the English language, as it fits the sounds produced by the actions so well. Not always the case.
My father’s sneezes always sounded like “HAIRPSH!”
I’m not convinced that the vocal cords are significantly involved in a sneeze, either. It sounds to me like just a very loud unvoiced sibilant.
Vocal cords slamming shut, then reflexively opening are part of what makes the sneeze happen.
Relevant wiki page:
See the “Sneeze” column in that table.
My vocal chords are not engaged when I sneeze; it just basically a large volume of air forcefully moving through a semi-restricted opening.
In turn, my sneezes sounds pretty much like ones of a large dog.
Sometimes, when I feel like being obnoxious, I engage my vocal cords during the “choo” portion of the sneeze. It’s like a hella loud shout on top of the normal plosive noise.
When my son sneezes it sounds like a deep bass “WHOOOORF” sound. With no “ah” to warn you he’s about to do a scary bear impersonation. 
I discovered long ago that you could actually vocalize when you sneeze. Somehow, I have no idea exactly how, I trained myself to exclaim, “Malahunga!” I only do it in private although I have slipped a time or two. No, I’m not joking.
Lovely…looks like most languages’ sneeze sounds are variations on “achoo.”
That’s how I’ve seen (heard) the Japanese force “hakushon” into a sneeze sound.
I say it’s a way to make the sneeze come out of your mouth instead of your nose. Because it can hurt when it comes out your nose. YMMV.
I have to be very careful not to holler “HORSESHIT!” during a sneeze should I ever find myself in polite company. Luckily, the dogs do not speak English and think it’s me asking if they want to play. Cuz Doge.