Generally the ear, and more specifically the cochlea
The ear is composed of 3 parts namely:
1. The outer ("external") ear, composed of the pinna and ear canal,
2. The middle ear, composed of tympanic cavity and three ossicles,
3. The inner ear, composed of semicircular canals, the utricle, saccule, and the cochlea.
Hearing happens initially by allowing the pinna of your ear to focus the sound. Then, by means of the ear canal and the ossicles, the sound is sent to the fluid-filled eardrum, which begins to vibrate. The vibrations are sent to the snail-shell looking component of the inner ear called the cochlea. The cochlea senses these vibrations and converts them to nerve impulses to be interpreted by the brain, making you "hear" the sound.