The part of the ear that vibrates first when sound waves enter the ear is the

The answer is eardrum (or tympanic membrane).

As sound waves enter the ear, they travel through the outer ear to the eardrum. By striking the eardrum, it begins to vibrate. Sound vibration is transmitted to the hammer (malleus), then to the incus and stapes. Stapes vibration moves the oval window and it is passed onto cochlea. The cochlea contains the receptor organ of hearing which translates the sound vibration into impulses which are then sent to the brain.