DS used to sleep during class when the kids were noisy and wake up at recess and lunch!! One of the teavhers at school has a very loud voice and yells fiercely sometimes, I taught next door to her for much of the pg and once when DS was tiny I took him into her classroom when she was yelling at the class, now this woman scares the smaller kids at school until they get to know her but Riley just cooed and smiled while she was yelling cause he knew the sound!! We joke it was a soothing womb noise to him!!
I have noticed with all my kids that the noise of lots of children playing and shouting doesn't bother them cause they heard it so much in the womb.