Quoc: In the auditory system, sound vibrations (mechanical energy) are transduced into electrical energy by hair cells in the inner ear. Sound vibrations from an object cause vibrations in air molecules, which in turn, vibrate your ear drum. The movement of the eardrum causes the bones of your middle ear (the ossicles) to vibrate. These vibrations then pass in to the cochlea, the organ of hearing. Within the cochlea, the hair cells on the sensory epithelium of the organ of Corti bend and cause movement of the basilar membrane. The membrane undulates in different sized waves according to the frequency of the sound. Hair cells are then able to convert this movement (mechanical energy) into electrical signals (graded receptor potentials) which travel along auditory nerves to hearing centres in the brain.

Where are sound vibrations converted into action potentials?
HW Chung: OK, some thoughts:
The relevant sentence in the passage is: In the auditory system, sound vibrations (mechanical energy) are transduced into electrical energy by hair cells in the inner ear. The answer is the inner ear.