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why would such a species have retained their ability to hear Because they still go outside to hunt, gather food and materials, and to interact with others of their species that live in other caves...
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#1: Initial revision
<blockquote>why would such a species have retained their ability to hear</blockquote> Because they still go outside to hunt, gather food and materials, and to interact with others of their species that live in other caves. To deal with the noise, they have evolved the ability to "turn down" their hearing in response to loud noise. When outside in the forest, hearing is quite useful. They "turn up" the volume when away from the loud waterfall. Whether this volume-adjustment is deliberate or involuntary is up to you, and could make interesting plot elements either way.