When you are over a mile up, it is noticeable at much less than significant distances.
The change in the speed of sound from sea level to one mile elevation is less than 2%.
http://www.grc.nasa.gov/WWW/K-12/airplane/sound.html
If you also have 30~40°F less temperature, you lose a couple more percentage points. The change is just not significant to believe you could measure the difference without measurement equipment.