A take I heard about how we explicitly got the 'sound from within the earth' when the truth is otherwise is actually just that the person who told us that was an Amaurotian.

Amaurotians are very bad at understanding immediately unfamiliar concepts (not to be confused with Concepts, which they think everything is), and nearly all of them haven't really considered what's going on outside the planet. So when they heard The Sound, they misattributed the source, because they didn't consider the possibility that something could be coming from outside the planet.