The Wilhelm scream is a frequently-used film and television stock sound effect first used in 1951 for the film Distant Drums. The effect gained new popularity (its use often becoming an in-joke) after it was used in Star Wars and many other blockbuster films as well as television programs and video games. Family Guy is one of those shows. The scream is often used when someone is pierced with an arrow, falls to his death from a great height, or is thrown from an explosion.
In "North by North Quahog", during the car chase, Peter avoids a piano in the center of the mall. Just before the priests working with Mel Gibson crash into it, the pianist jumps out of the way screaming.
In "Long John Peter", during the pirate duel between Griffin, his band of pirates, and Shelley Boothbishop, the cannon in Peter's car accidentally fires upwards, breaking a utility pole in half and the famous scream is heard as the worker on it falls.
In Something, Something, Something, Dark Side, the scream is heard when a rebel is shot during the battle on Hoth.
In "Amish Guy", the scream can be heard when the roller coaster crashes due to Peter's weight.
In "Internal Affairs", the scream is heard during the chicken fight, when Peter and the Giant Chicken travel back in time to the days of cowboys and Indians, and they toss a cowboy overboard.
The scream can be heard during Peter and Homer's fight in "The Simpsons Guy".