When sound propagates in the lower atmosphere it can travel through denser or thinner parts of air. This could cause a "portion" of the sound to bend upwards, making it harder to hear. 

Factors that could cause the bending are: wind speed and/or temperature that change based on altitude. 

For more info you can check this [link][1]. 


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In the experiment you describe, night and day would have an effect on both temperature and winds, therefore can potentially cause the sound to be refracted upwards, changing the sound intensity reaching the observer. 



  [1]: https://en.wikipedia.org/wiki/Wind_gradient#Sound_propagation