What different meanings can you give the story's title?
The phrase “a sound of thunder” is used as a metaphor twice in the story. The first occurrence is when the hunting party is back in prehistoric time, and the Tyrannosaurus rex bursts onto the scene in the jungle. The creature is so immense, so powerful, so overwhelming (especially to Eckels), that the noise it makes is equal to what we consider to be the loudest sound on earth: that of great thunder. It takes over the landscape. There seems to be no escape from it. In this instance, the phrase refers to the dinosaur.
The second occurrence of the phrase comes with the last line of the story. The hunters have returned, Eckels has done something he shouldn’t have done, and the future, now the present, has changed as a result. Time Safari guide Travis is furious. We read that he clicks off the safety catch on his rifle and begins to raise it. Then comes “a sound of thunder.” This time, it refers to a gunshot. The men are in the small office of the Time Safari business. A shot from a rifle would resound greatly in such a confined, metallic-surfaced area. It would be just as overwhelming and powerful as a large clap and after-rumbling of thunder would be, outside. Whether Travis shot Eckels or himself is a consideration for another question.