DEMO: You can use a Slinky to get a good idea of how compressional waves, such as sound, move. Stretch a Slinky out and then give one end a good strong knock or jiggle. This will push the coils near your hand into the ones next to them, which will be pushed into the next and the next and so on, all the way to the other end. At the other end, the compression will rebound and move back in the other direction. Thus a "compression" moves back and forth, comparable to a sound wave and an echo. In sound waves, it is molecules of air that get pushed into each other, rather than coils of a spring, but it is the same concept. Of course, sound will also travel through water and even solid materials (how else could you "hear through walls"?). Even in this case, the sound waves move by pushing molecules forward in the direction of motion.