Why do you think scouts could hear better through the solid ground rather than

through the air?

Sound travels better through solids than through air.

Explanation:

Think about being in the pool: you can hear a lot of things, albeit muffled a little. Or think about tapping a really long pole, and someone standing far away can hear the vibration or the noise while touching the pole.

Solids are more tightly packed together. Sound needs a medium to travel though, and anything with densely packed atoms is better than loosely packed. The hierarchy is solid > liquid > air. Kinda funny that we do most of our talking through air.

Sound is better heard through solid ground then air