Hyperacusis in Bell's palsy is due to the paralysis of the following muscle -
Stapedius muscle dampens excessive vibrations of the stapes caused by high pitched sounds in order to protect the internal ear.
If this protective reflex is not elicited, it indicates stapedius paralysis and results in hyperacusis.
Tensor tympani
Levator palatini
Tensor veli palatini
Stapedius