I also hardly make a sound during childbirth. First time I muttered a few times 'I can't do this' (ended in a C.section) Second time just a long grunt as the head came out (MW said 'Its ok, it's just baby) & a longer grunt as the body came out. Stung like hell.
The only time I've ever screamed as an adult involved my poor pet rabbit, & some maggots, & a wound. That's all I'll say.
I was actually pregnant at the time & I let out this HUGE scream that seemed to come from deep in my soul, & also fell to the floor in shock. & It's the only time I've ever done that too. Poor bunny