I don't get the whole thing against vocalizing while working out. Probably not helpful if you're doing something that isn't particularly strenuous, but if you are pushing or pulling heavy weight, or forcing out extra reps beyond your "normal" range, then it definitely helps focus your physical exertion. In the martial arts there's something similar called "ki-ai" that you are expected to do. If I'm deadlifting 400+ pounds, then I am not going to do it quietly. The only way I can physically do it is if I let out a grunt.
Kor
If you're in a gym you're not alone and you're bothering people. Some do it intentionally aka "Look at me and how awesome i am!"
With Martial Arts you would be usually in a Dojo and it's expected there, in a gym it's considered rude.