Learn Russian: Там, где покрытие не поглощает шум, он сильно отражается от стен. - Where the coating does not absorb the noise, it bounces strongly off the walls.

This is a classic Russian spatial correlative clause. While English just says "Where X happens, Y happens," Russian pairs a demonstrative adverb (там - there) in the main clause with a relative adverb (где - where) in the subordinate clause. You can think of the Russian structure literally as "There, where the coating does not absorb noise, it bounces."

Does он refer to the coating or the noise?

It refers to шум (the noise), which is a masculine noun. If it were referring to the coating (покрытие), the pronoun would have to be neuter (оно). In Russian, tracking the grammatical gender of nouns makes it perfectly clear exactly what is bouncing off the walls, even though both nouns translate to "it" in English.

Why is the reflexive verb отражается used here instead of отражает?

The non-reflexive отражает requires a direct object because it means to reflect something else (like a mirror reflecting light). Because the noise isn't reflecting another object, but rather performing the bouncing action itself, we add the reflexive -ся ending to make the verb intransitive.

What case is стен in the phrase от стен?