Answer
See explanation.
Work Step by Step
At night, the cooler air near the ground results in slower sound waves, which makes the air akin to a high index of refraction material. Sound waves heading upward will bend away from the normal/vertical when they encounter the interface with warmer air above, because the sound waves travel faster there.
This bending is back downward, toward the Earth. In other words, sound waves that might otherwise have escaped the atmosphere are instead refracted down, enabling a faraway listener to hear them.
