you seem very certain soft (volume) speech can't harm. okay then you should be fine with this experiment (and confident about its result): let me put you into a room and have people come in for 8 hours a day and call you stupid, ugly, worthless, a disappointment, etc. in perfectly mild tones for exactly one month. we'll perform a mood/outlook/mental health assessment before and after. do you think there will be a difference in the two assessments?