Why is the speed of sound faster in a humid and warm sea-level rainforest compared to the top of Mount Everest?

Because the speed of sound varies depending on the medium and its temperature.

Explanation:

Sound is a mechanical wave, this means its velocity of propagation depends on the medium.

Now, in general sound waves travel faster in solids than in liquids or gases. In addition, it varies with changes in the temperature of the medium. This is because an increase in temperature means that the frequency of interactions between the particles that transport the vibration increases, hence this increase in activity increases the speed.

So, the speed of sound in a gas (air, for example) is not constant, but depends on the temperature.  

Hence, the higher the air temperature, the greater the velocity of propagation; that is why the speed of sound is faster in a humid and warm sea-level rainforest compared to the top of Mount Everest, which is cold.