The speed of sound generally decreases with increasing altitude. This occurs because:

The speed of sound is closely related to the temperature of the air through which it travels. As altitude increases, the air temperature typically decreases in the lower atmosphere (up to the tropopause). Sound waves travel faster in warmer air due to the increased kinetic energy of air molecules. Consequently, when the temperature drops with altitude, the speed of sound decreases as well. This relationship is a fundamental aspect of aerodynamics and is critical for pilots to understand when considering the performance of aircraft at different flight levels.

The other factors, such as air pressure and composition, do play roles in atmospheric science but do not directly account for the change in the speed of sound with altitude in the same way that temperature does. Humidity can affect sound speed somewhat, but its impact is relatively minor compared to the primary influence of temperature.