Answer:
Milk
Hope this helps,if not sorry
Static friction - the friction that occurs when a force is being applied on a still object.
Sliding friction - the friction that occurs upon a moving object; it is always lower than the static friction of the same object.
Rolling friction - the friction on a rolling object; it is actually a form of static friction with the contact point of the wheel with the surface.
Because the sound waves are being observed by the people that are there and it is not empty as in an empty place the sound waves will come back to you.