A cacophony is a mishmash of unpleasant sounds, often at loud volume. It's what you'd hear if you gave instruments to a group of four-year-olds and asked them to play one of Beethoven's symphonies.
A cacophony is a jarring, discordant mix of sounds that have no business being played together. When the orchestra tunes up before a show, it sounds like a cacophony because each musician is playing a completely different tune, at different times, and at different volumes. Once the show begins, that cacophony had better turn into a melody, or audiences will demand a refund. This allegedly occurred during the first time Igor Stravinsky's score for the ballet "The Rite of Spring" was performed because its difficult composition and discordant tone was shocking.