Whenever something / someone produces sound waves, it first goes into the canal of your ear, with the parts of the outer ear guiding it into there. Then, it hits your eardrum which vibrates your malleus, sending a reaction through the stapes and incus and into your cochlea. Then, the cochlea sends signals through thousands of bundles of hairs, which each pick up a different pitch depending on where it is inside the cochlea. After that, the hairs create electrical signals, which they send through the auditory nerve, into your brain. Finally, your brain registers it, and that is how you hear sound.