This drawing shows graphs of two sound waves on top of each other. They have the same wavelength and amplitude.
Sound waves can get into this situation a variety of ways:
In all cases, the resulting sound is the sum of the two waveforms. Since the bottom loops of the wave represent negative quantities, the result may actually be less sound than either wave would give by itself. In fact if the waves were placed just right, there may be no sound at all!
That magic effect would happen if the waves were exactly one half wavelength off. We often talk about waves as if they were circular (it's a handy way to describe something that happens over and over) and call this situation 180 degrees out of phase.
All of the causes listed involve a relationship between distance and wavelength. Can you see that if the frequency changed, but nothing else, you could wind up with any amplitude up to twice the original?