Answer:
I pass before the sun, but make no shadow.
I can be quick and then I'm deadly, I am a rock, shell and bone medley. If I was made into a man, I'd make people dream, I gather in my millions By ocean, sea and stream.
Upon me you can tread, though softly under cover. And I will take you places, that you have yet to discover. I'm high, and I'm low, though flat in the middle. And though a joy to the children, adults think of me little.
Tall in the morning, short at noon, gone at night. But I'll be back soon.
What has to be broken before it can be used?
I repeat only the last word you say. The more I repeat, the softer I got. I cannot be seen but can be heard. What am I?