Since “A Sound of Thunder” is the title, and the phrase is used twice in the story, these words must be considered important to us. They hold the key to understanding the theme. The first time they appear is when the Tyrannosaurus rex shows up in the jungle. It is so large and so scene-stealing that the sound it makes is enveloping – just as thunder pounding and rumbling during a storm can sound and feel as if it surrounds you. It is at this point that the hunter Eckels becomes so terrified that he must stumble back to the time machine without firing his gun.
The second time the phrase appears is in the final line of the story. The safari team and the hunters have returned to the office, only to learn that Eckels’ steps off the path have changed the course of history and have impacted the future. Without saying a word, lead guide Travis lifts his rifle. “A sound of thunder,” is how Bradbury succinctly ends the story. By these words, we infer that someone has gotten shot, and that the sound has reverberated throughout the confined space boxed in by the Time Safari office walls. But who did Travis kill: himself, or Eckels? In either case, we understand the ramifications of time travel and its ability to change the future.