Q:

"Sound can be heard better at night then by day"- Rashi

Rashi writes in Masheches Beitzah 7b on the gemara about different cases of when we consider an egg found in a nest a fertilized or unfertilized egg. The gemara talks about a case when a hen can hear a roosters crow by day. Rashi writes on this that sound can be heard better at night then by day. Tosfos also makes this point.
My question is does this theory match modern day science,and was this a scientific belief in Rashi's times or was this a novel theroy of Rashi?
Rashi's words: שהקול אינו נשמע ביום כבלילה 
Tosfos words: ובלילה משמיעין קולם למרחוק יותר מביום...

A:

I can't say how much Rash"i might have been influenced by science in his statement. However, it is scientifally true that sound waves do travel further at night.
Excerpt from here:

Actually, sound transmits farther at night may be related to
  refraction of sound waves! First, sound is the vibration of air, and
  it is a kind of wave motion. The propagation of sound wave is faster
  in hot air and slower in cold air. Therefore regions of air at
  different temperatures have different refractive indices, just like
  media with different optical densities. When sound wave propagates in
  air whose temperature changes with altitude, refraction of air
  happens. Sound will move towards areas with lower temperatures.

(BTW, this also largely explains the "calm before the storm." It is more than just a metaphor. It occurs, partially because of the sudden change in air temperature as well as, partially, darkness.)
I can't say when, exactly, understanding of heat, sound, and thermodynamics were discovered. But, I'm quite certain that these concepts as described above were known far before Rash"i's time.

A:

Rashi's source is a Gemara in Yoma (20b):

דא"ר לוי מפני מה אין קולו של אדם נשמע ביום כדרך שנשמע בלילה מפני גלגל חמה שמנסר ברקיע כחרש המנסר בארזים
ת"ר אלמלא גלגל חמה נשמע קול המונה של רומי ואלמלא קול המונה של רומי נשמע קול גלגל חמה ת"ר שלש קולות הולכין מסוף העולם ועד סופו ואלו הן קול גלגל חמה וקול המונה של רומי וקול נשמה בשעה שיוצאה מן הגוף
As Rabbi Levi said: Why is a person’s voice not heard during the day in the manner that it is during the night? It is due to the fact that the sound of the sphere of the sun traversing the sky generates noise like the noise generated by a carpenter sawing cedars, and that noise drowns out other sounds.
The Sages taught: Were it not for the sound of the sphere of the sun, the sound of the bustle of the crowds of Rome would be heard throughout the world; and were it not for the sound of the bustle of the crowds of Rome, the sound of the sun’s sphere would be heard throughout the world. And the Sages taught: Three sounds travel from the end of the world to its other end, and these are: The sound of the sphere of the sun, and the sound of the bustle of the crowds of Rome, and the sound of the soul at the moment that it leaves the body,

