"The sound of thunder" occurs five times in Ray Bradbury's short story of the same name: four times in reference to the Tyrannosaurus Rex, and once in reference to a gunshot. The sound of thunder is always preceded and followed by silence, so that it erupts into the senses of the characters and the reader. It denotes a sudden, dangerous change in the environment, the way actual thunder announces the arrival of a storm.
In the story, "the sound of thunder" is first mentioned when the Tyrannosaurus appears in the distance. Mr. Eckels, the protagonist, has paid a time-travelling safari company a huge sum of money to take him to the distant past so that he can hunt a dinosaur. Eckels is a big game hunter, and he is thrilled to have the chance to hunt the biggest game of all. When the group arrives in the Cretaceous period, Eckels looks around him with joyful anticipation:
The jungle was high and the jungle was broad and the jungle was the entire world forever and forever. Sounds like music and sounds like flying tents filled the sky [...] Eckels, balanced on the narrow Path, aimed his rifle playfully.
“I’ve hunted tiger, wild boar, buffalo, elephant, but now, this is it,” said Eckels. “I’m shaking like a kid.”
There are extremely strict rules about what tourists must not do in the past, but Eckels is far too excited to take these rules very seriously. Then his dinosaur appears above the canopy of the jungle:
The jungle was wide and full of twitterings, rustlings, murmurs, and sighs.
Suddenly it all ceased, as if someone had shut a door.
A sound of thunder.
The enormous Tyrannosaurus strides towards the safari group, and Eckels's excitement turns into stark terror. The dinosaur is a force of nature, described as tons of "meat, ivory, and steel mesh," with "teeth like daggers" and eyes "empty of all expression save hunger." It crushes everything in its path. Eckels is too frightened to shoot, and the safari guides tell him to go back to the time-machine while they take down the monster. In his terror, Eckels stumbles off the path that separates the time-tourists from the world of the past while behind him, the guides struggle to kill the dinosaur.
The rifles jerked up and blazed fire. A windstorm from the beast’s mouth engulfed them in the stench of slime and old blood.
The thunder of the dinosaur's arrival is now mixed with the lightning strikes of gunfire and the wind of the dinosaur's roaring. The storm rages behind Eckels as he staggers back to the time-machine:
The rifles cracked again. Their sound was lost in shriek and lizard thunder. The great level of the reptile’s tail swung up, lashed sideways. Trees exploded in clouds of leaf and branch.
More gunfire rings out as the guides shoot the Tyrannosaurus in the eyes, and
[like] a stone idol, like a mountain avalanche, Tyrannosaurus fell. Thundering, it clutched trees, pulled them with it.
Fountains of blood, like sheets of rain, drench the guides, and then abruptly the storm is over:
The thunder faded. The jungle was silent. After the avalanche, a green peace. After the nightmare, morning.
However, Eckels has broken the rules of time-travel by leaving the path, so although the storm of the dinosaur's shooting has ended, another, greater storm has been unleashed on time. When the safari group return to the present day, they find the world subtly but definitively altered. Their arrival in the the present is like their arrival in the past, where they stand looking around at the environment for a minute, gauging it, before sensing that something is wrong. Silence falls as they realise that everything has changed:
[Eckels] stood drinking the oddness with the pores of his body. Somewhere, someone must have been screaming one of those whistles that only a dog can hear. His body screamed silence in return.
In horror, Eckels falls to his knees, too terrified to move, shivering with his eyes screwed shut and hoping the storm will pass him by. The lead safari guide raises his gun, and there is a final "sound of thunder." The story ends, but given the previous nature of the "sound of thunder," which opened the scene of death and destruction with the Tyrannosaurus, this final sound of thunder is merely the opening salvo of another storm—a storm caused by Eckels's irresponsible behaviour in the past.
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.