Hearing doesn't actually improve per se, it's just that sight is completely dominant over all other sensory modalities. If you see something but feel it differently, you'll believe what you see even if it is wrong. So I think it's just when you don't have sight anymore, you rely on hearing etc more, but you're not actually better at it. Here's something interesting though, if you lose a limb, after a couple of years the neurons in the brain that were responsible for the sensory and motor functions of that area are re-specialised to other areas. Also, it depends where your sight is lost. If you've got a problem with your eyes, then the area of the brain responsible for vision is still active so it might get respecialised, but if that brain area is damaged, resulting in loss of sight because visual information isn't processed, then of course those neurons are dead so can't be respecialised.
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Goddess of Undress
Co-owner of the Evil Petting Zoo and Sorceress of the HADB
OLD COOTS eye candy and Disrobed Division of the ORT
And her sidekick Pink