How "achoo" is actually pronounced during a sneeze

It's a myth that "achoo" is pronounced "aah-CHOO" with the emphasis on the "choo" part. The way the word was originally invented, it's actually "ACH-oo". "ACH" is the big expelling factor and "oo" is a very quiet sub-vocalization after the main part of the sneeze.

A sneeze generally has 3 parts: Inhale, big expelling ending in "sh", and sometimes a very quiet vocalization after the "sh" as the air expelling draws to a close. The big expel always has an "sh" sound. The loudest sound is the fast-moving air through the "sh" syllable. This is universal and it's literally impossible not to do it while sneezing. So it is simply not possible for "oo" to be part of the loud sound, unless of course one were to tack it on voluntarily after sneezing. Anyone sneezing with no cultural background (e.g. babies, small children), will form a sound resembling more "ASCH-oo" than "aah-CHOO". In fact literally the only people who pronounce it the second way, are people who learned the word "achoo".

Observing the full list of sneeze sounds in every language, it appears the majority including put emphasis on the 1st syllable while a few including English put emphasis on the last syllable. It is my hypothesis that unfortunately at some point in history, the onomatopoeia became misinterpreted. Now a lot of children's books and cartoons depict people saying "aah, aah, aah-CHOO!" and the unnatural pronunciation is stuck in culture.