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

through the air?

Sound is better heard through solid ground then 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.