What is the signifcance of Miller ending the play with the line "the drums rattle like bones in the morning air"?

This line creates a powerful image in the mind's eye. As audience or reader, we imagine the similar sounds that drums and bones make. Miller connects these two images because he is trying to show that our memories of an event like this can be called up by any number of sounds or sensory experiences. The image of bones refers to those innocent people who were killed needlessly. "Bones in the morning air" is a reminder that such a sound memory can return on a daily basis; it is a truth that will never fade and should always be remembered. He wants us (the reader or audience) to recall these events as we would a common sound in nature, since they are a part of our common humanity.