As an example of sound traveling underground, here in the
battleground state of PA, there are a lot of places where there are
streams underground. A tiny trickle of water can be heard even by my
bad ears, several feet underground.

One little thing in my situation, we're about 1/3 mile away from the
Pacific North West Ocean surf and believe me, it is genearlly quite
loud.... enough to preclude hearing delicate sounds in the soil at times.
It took a relatively quiet day just to hear the fish tape rattling.

Ed :^)