Master Uechi is said to have instructed that students should do what comes naturally and be patient, as it will teach itself. Keep in mind that there is no right way, just a right way for you. Forget imitating the hissing noises classmates make for a while. Simply push your stomach muscles out and down to make them hard but not quite rigid. Now minimize your breaths.
Shorter breaths mean even less vulnerability to a strike.
The kiai represents karate's roar, the all-out strike. Try dragon breathing during your sanchin opening too. You should breathe during your kata between, not during movements. If sanchin is performed as a whole-body exercise, breathing after each movement adds a meditative feel that clears the mind. If the kata is performed for power or speed, then breathing less frequently makes sense.