Perhaps it's simpler to think of the other way - why the speed of sound is in marbles is the same as the speed that a marble would pop out of the tube when anot

Perhaps it's simpler to think of the other way - why the speed of sound is in marbles is the same as the speed that a marble would pop out of the tube when another was inserted. Imagine that instead of pushing a whole marble in you merely wiggled the last one rapidly.