Why are sounds louder at night?

i think it might be because there are fewer sounds to mask the source. normally in the daytime there is a lot of vehicular traffic that can mask many sounds.

another possibility is that once the sun is not heating the air near the surface of the earth comparitively the air higher up is warmer. this has an effect of a temperature gradient where sound propagation originating from near the earth surface is refracted back down to the earth once it reaches the higher warm air. this allows the sound to travel further with less attenuation.