Question

why is hearing affected while going up or coming down from hills ?

Answer 1

Cause intensity of sound depends on density of surroundings and density of air changes as we move up or down the hill.

Answer 2

the air is rarer when you go up and gets denser when you come down
as we all know , sound needs a medium to propagate, so sound travel differently while going up or coming down from hills
the second reason (your own body)
the atmospheric pressure decreases when you go up the hill, the blood pressure gets affected resulting pain in ears due to more blood getting supplied to ears that might affect your hearing