Hearing can fail in many ways, and to some extent, becoming hard of hearing is an unfortunate but inevitable consequence of normal ageing processes. Given how much our society relies on verbal communication, losing the ability to hear clearly can be devastating.
Thankfully, much progress has been made in recent decades in the area of auditory prosthetics: hearing aids and cochlear implants. The science behind these devices is very interesting, and is discussed in chapter 8 of "Auditory Neuroscience" . The following web pages provide supplementary material.