Answer: The different speed of sound waves in water and in air
Step-by-step explanation:
In general, sound (which is a mechanical wave) travels faster in solids than in liquids or gases. This is because the speed of mechanical waves is determined by a relationship between the elastic properties of the medium in which they are propagated and the mass per unit volume of the medium (that is: density).
In other words: The speed of sound varies depending on the medium through which the sound waves travel.
This is why Jason notices that sounds underwater (a liquid medium) are different from sounds above water (a gas medium: air).