From a physics point of view, there are more particles in the air at night. Since there are more particles in the air, the air is more dense and carries the sound waves faster and farther.
There is also the factor of less sensory input, I.e. Your eyes. Visual senses take up massive amounts of "brain usage." Case in point: How far can you see during the day vs at night? Your brain is processing data received by touch, taste, smelling, hearing and seeing. Remove ONE of these senses and the others will increase slightly. Try this: Go outside during the day an close your eyes. You've fooled your brain into thinking it's night time (no visual sense). You'll be amazed what you hear. Now, eat an apple. BOOM! Your hearing seems to decrease because your brain has something else to work on. While eating the apple with your eyes closed, smell some flowers or pet a soft animal. Boom! Your brain has even MORE input to worry about! In reality, you are hearing all the background noises, you're just sending more data to your brain.
Sound operates a lot like gravity, the more obstructions, the less we hear..i.e. How in office bldgs. They will use white noise, often fountains, etc. To help filter out distractive sounds. The less activity, the further the sound waves can travel. In the north, the colder it gets, the further sound travels. Neat eh!
We relay more on sound cause there is only limited visibility and we don't have night vision like other animals
Could sound waves be affected by the temperature in the air?
Because it is much quieter at night.
It might be because it is considerably quiter at night.