Even in a completely cleared burial chamber you still have those sounds as ambient noise. Cleared burial chambers can still act as a spawn point and can produce skeletons in the outside environment.

You'll notice the same thing happening in cleared out troll caves in the black forest, crypts in the swamp (except they'll have dragur and blob sounds) and cleared out frost caves in the mountains (with the same caveat about biome specific creature sounds.)

You will also notice that the swamps have leech sounds near the water even if there are no leeches present. Just like you will hear branches snap in the meadows and black forest even if there are no creatures that you can interact with present.