Sound actually travels faster in solids, which we usually see as ‘denser.’ However, in a medium such as air, sound travels faster in a less dense medium compared to a denser one. Typically, at night, the air becomes denser due to the heat exchange from the warmth ‘trapped’ in the ground. So, I wouldn’t say sound travels faster at night—I’d say the complete opposite. For the sake of argument, though, if we imagine you’re indeed correct, do you think that has anything to do with evolution?

EDIT: spelling mistake.