A cacophony is a harsh, discordant mixture of sounds; a din, racket, noise, clamor, discord, dissonance, discordance, uproar.
In The Great Gatsby, the best examples of cacophony come in Chapter 3, during Gatsby's party. The atmosphere is like a Roman Carnival: a bunch of drunk people, couples fighting, a full orchestra, out-of-control dancing, cars crashing, and lots of booze clanking about.
Here's a sample:
There was dancing now on the canvas in the garden; old men pushing young girls backward...and a great number of single girls dancing individualistically or relieving the orchestra for a moment of the burden of the banjo or the traps. By midnight the hilarity had increased. A celebrated tenor had sung in Italian, and a notorious contralto had sung in jazz, and between the numbers people were doing “stunts” all over the garden, while happy, vacuous bursts of laughter rose toward the summer sky.
Later, we hear the car wreck:
However, as they had left their cars blocking the road, a harsh, discordant din from those in the rear had been audible for some time, and added to the already violent confusion of the scene.
In this chapter, Fitzgerald vividly describes the reckless and careless behavior of the party-goers. The car accident will foreshadow the one later in the book when Daisy runs over Myrtle. Like Nick, we are being introduced to the decadence of the Jazz Age.