The speed of sound differs depending on what it is traveling through. Sounds move more quickly through higher density materials than lower ones. At sea level, sound travels at a rate of 340.29 meters per second (768 MPH. A sonic boom occurs when something travels faster than this), while underwater, it travels at a rate of 1497 meters per second. About 4.4 times faster. Sound moves more quickly in higher density materials such as water because,the particles are much closer together, making it easy to spread disturbances in the pressure of the particles. In the hippos case, the ones that are underwater will hear the communications before the ones above water will.