At night, the atmospheric air becomes moist than in day time. So, the density of air at night is less than at day time. But the velocity of sound is inversely proportional to the square root of density i.e., \(v\propto\frac1{\sqrt\rho}\).
So the velocity of sound at night is more than at daytime due to the reduced density of air. Hence, the sound made at distance can be heard distinctly at night than in the daytime.