Back then you couldn't even look at the eardrum.

It's acoustic vibrations in the air, little waves of energy in the air that tickle our eardrum.

Somehow in tickling our eardrum that transmits energy down our hearing bones, which get converted to a fluid impulse inside the cochlea and then somehow converted into an electrical signal in our auditory nerves that somehow wind up in our brains as a perception of a song or a beautiful piece of music.