This library Sci Fi book I read in the early eighties, in the UK. I think it was hardback, but I am not sure.
There is a planet with a massive, miles and miles high, mountain range. The range has a system of gigantic dark caverns that are home to really horrific looking bat-type creatures.
The bats are the source of a life-prolonging (I think) drug and are hunted for this drug. The hunters are various tribes of genetically blinded people who are more or less slaves. They live in the caverns in scattered villages, and they put their daily quota of bats onto something like transporter plates; in exchange a load of food appears.
The people are blind because (I think) the bat-things are so scary-looking, but also so everything can stay gloomy and dark to not disturb the bats at breeding times. They hunt in little gliders with long harpoons and are trained to fly them from an early age so they can hunt from their late teens onwards.
The protagonist is a girl approaching adulthood and her first hunt. However, she has eyes that work, although on her birthday it's traditional to remove them. Her father is often urging her to find a mate from the next village (I think they do this to avoid inbreeding), but she isn't ready yet.
The kids in her village play handball often, but they have bells inside the ball to hear it. They can't comprehend why she's so good at playing, because she can see the ball in the gloom.
I think some people on a quest (maybe for the source of the geriatric drug) visit this world, where one side of the mountains is always cold and dark because of the height. They somehow make contact with this girl, and she emerges into daylight on the sunny side of the mountains.
They notice she walks with her head erect and alert like a blind person listening for location clues, because that's how she first learned to walk. Then there is an adventure with this party, but I can't remember that part; the 'bat cave' is my main memory of the book.
My update: I've done some hard thinking on this one, a further memory, there are also peasant economy villages spread around the sunny side of the mountains, they are sighted, hard working farmers. They object a lot to growing all this food and have to watch it disappear into the mountains and all they get in return is weird dead giant bats which are loaded up for starships...
Edit: Thanks to Ayshe for identifying this story as 'Light of the Sound'! Once I had the title I found a bit more info on an old website.....it appears I was wrong about the bat creatures being wanted for immortal drugs, it was actually starship drives
Extra info from Thierstin net :-
The story plays of on the world of Gallendys, where the boy Kelver lives in the shadow of the Skywall – actually one side of a crater 100k (klomets, in the book) high, and thousands wide. Inside it live, we learn, an ancient, pre-human race of flying beings (called 'Delphinoids' by the Inquest who rules the human dispersal). Also inside the skywall exist tribes of engineered humans – deaf and blind, so they don't have to see and hear the lightsongs of the Delphinoids which they call Windbringer.
The Delphinoids don't see into our reality – the see the Overcosmos, and, if muted, mutilated, and soldiered into a spaceship, can pull it through the Overcosmos to anywhere the Inquest wishes. So they need to be hunted, flash-frozen, and delivered to the shipyards – but no human who has seen and heard them sing can do so; thus the specially bred race to do the dirty deeds. To this race the girl Darktouch is born – a throwback, who can see & hear. And, by chance, before the initiation ritual where she would have lost these senses she sees the lightsongs, which compels her to flee and escape the Skywall – which leads to her meeting Kelver in the process
