Why is this the answer?

The answer is leaves because this clever riddle uses a special trick with words to describe the leaves you see all around you, especially in the fall. It talks about leaves when they are still alive and leaves after they have fallen from the trees. Let's look at the first line: "Walk on the living, they don’t even mumble." The "living" part of the riddle is talking about the green leaves that are still attached to a tree or bush. Think about walking in a park during the warm months. Even when the leaves are moving on the tree from the breeze, they make a soft, whooshing noise that is very calm. The riddle says they do not "mumble" or complain when you walk near them because they are still part of the strong tree, quietly doing their job of making food for the tree from the sun. If a leaf is alive and healthy on its branch, it is silent, so when you walk under it, you do not hear a sound or a 'mumble.' Now for the second part: "Walk on the dead, they mutter and grumble." The "dead" part means the dried-up leaves that have already finished their job for the year. They have turned brown, yellow, or red and dropped onto the ground for the winter. When you walk through a big pile of dry, fallen leaves, what fun sound do they make? They crunch, crackle, and rustle loudly. The riddle calls this sound 'mutter' or 'grumble' because it is a low, whispering noise, like the leaves are complaining or talking back to you as you step on them! They are no longer soft and flexible like they were on the tree. Because they are dry and brittle, they break easily, which creates that noisy sound. The old, dry leaves on the ground are the 'dead' ones, and they are the ones that make a wonderfully loud, crunchy noise when you step on them. That's how all the clues fit together and show us that the answer is leaves.