There could be a few reasons for this. One possibility is that there is reduced air pressure at higher altitudes, which can lead to decreased air density. Since sound travels through the vibration of air molecules, the reduced air density at higher altitudes can result in less efficient sound transmission, making it harder to hear sounds clearly.Additionally, at higher altitudes, there are typically fewer obstacles and less ambient noise, such as traffic or human activity, which can make sounds seem fainter in comparison.Furthermore, the temperature and humidity levels at higher altitudes can also affect how sound travels. Changes in temperature and humidity can alter the speed at which sound waves propagate through the air, potentially resulting in a muffled or distant sound perception.