A person's eardrum vibrates normally, but the ossicles do not transfer vibrations effectively. The cochlea and auditory nerve are undamaged.
Which explains why the person has difficulty hearing?
Less sound reaches the eardrum, so its frequency becomes lower.
Less vibration reaches the cochlea, so weaker signals are produced.
More vibration reaches the cochlea, so all signals are stopped.
More sound reaches the eardrum, so its frequency becomes higher.