Mechanical energy of vibrations is transduced to the electrochemical energy of neural impulses at the:

COCHLEA

This is a portion in the inner ear that receives sound in the form of vibrations which cause the cilia in it to move. The stereocili then convert these vibrations into nerve impluse which are taken up by the brain to be interpreted.  Two of the three fluid sections are canals and the third is a sensitive organ called Organ of corti which can detect pressure and impulse travelling to the auditory nerve.