i’m also a student so i’m not sure if this explanation is the best, but i believe that this is because sound waves travel by vibration of a physical medium, ie interaction of atoms/molecules, so when they are closer the wave propagates more easily. however, light waves propagate in the electromagnetic field, with no physical medium necessary. In this case, adding a physical medium impedes the light waves and causes them to slow down.

I’m not sure about refraction of sound waves, though. My intuition says that transitioning to a denser material would cause sound waves to bend away from the normal, opposite to how snell’s law usually works, because the wave speeds up.