Why do people say "ooh-wee," "oh-way", or "ay" when sparring in Muay Thai? I know what it means, however it keeps the mouth slightly open, and should be Closed for single sided mouth guard for safety / prevent teeth damage. Is that correct? In boxing, they recommend people whistle out air with mouth closed "shhh". Just curious, thanks
As I understand it, the boxing exhalation, as much through the nose as through the mouth, is a function of the mouthguard itself (I've seen some references to a required form factor for mouthpieces and others saying it's entirely unregulated. I'm not a boxer myself). Function follows form.
As regards the Muay Thai noises, it basically works a lot like the Japanese kiai, the Korean kihap, or the like. As a forceful exhalation, it teaches fighters to tighten their core with their strikes, it reminds them to breathe by force periodic evacuation of air, and it focuses the mind on that final moment of the strike. Lastly, there is an aspect of calling the hits for scoring. Shouting while striking serves as a signal to the judges of "here's a hit for you to score" and the corresponding grunts when hit is partly a carryover of the acknowledgment in training that a hit was scored (since there often is not a judge) and also as a dismissal of the hit, basically indicating "Yeah, that hit, but it didn't really hurt", which might influence whether the hit is scored.
As you note, an opened mouth can be dangerous, particularly for the purpose of keeping a mouthguard in. Therefore, I've seen numerous recommendations, including from Muay Thai practitioners and teachers, to go for more of a closed-mouth grunt, including some opining that it can lead to a positive habit of the grunting yell also involving biting down on the guard at the same time, reminding the fighter to do that step to protect from a counterstrike.