One thing I would keep in mind is. That it was sound that made their hearing go bad, so what you want to do is take the sounds they can hear and make them more audible. That will degrade their hearing even more.
There is new technology's "Auditory brainstem response" that is being used today.
But that will cost money and if your not willing to put any money into it your best bet is to just use some equalizer to tune the sounds to what is audible to the subject. Having hearing damage then their range of sounds will be limited, so an equalizer would be the best bet in this case.