Hyperacusis in Bell's palsy is due to the the paralysis of the following muscle:

A complete interruption of the facial nerve at the stylomastoid foramen paralyzes all muscles of facial expression. If the lesion is in the middle-ear poion, taste is lost over the anterior two-thirds of the tongue on the same side. If the nerve to the stapedius is interrupted, there is hyperacusis (sensitivity to loud sounds). Lesions in the internal auditory meatus may affect the adjacent auditory and vestibular nerves, causing deafness, tinnitus, or dizziness. Intrapontine lesions that paralyze the face usually affect the abducens nucleus as well, and often the coicospinal and sensory tracts.