Summary: his theory is that it is similar to the sound of you damaging your teeth, since teeth have no nerve endings, the sound of them being damaged makes you cringe.
And he surmises that the sense of hearing evolved from the need to protect teeth. Seems that nerve endings in teeth would have been more likely to evolve.
More nerve endings in teeth mean it's harder to eat if you get decay or are eating something hard. Given the state of dental hygiene back in the day, it makes some sense.
