"Stars and Stripes Forever" is a common song used to celebrate victories and events. It was used in the episodes "Summer Sucks", "Free Hat", and "Douche and Turd."
Background
In "Summer Sucks," Mayor McDaniels asks for the South Park Elementary orchestra to play the opening of the Fourth of July ceremony. The song is played very badly due to the student band not practicing or knowing the song beforehand.
In "Free Hat," instead of being awarded for the defeat of George Lucas and Steven Spielberg's updated version of Indiana Jones: Raiders of the Lost Ark, the boys are presented with a celebration for getting baby killer Hat McCullough pardoned.
In "Douche and Turd", the music plays while Cartman pedals a float with Butters on it, dressed as a hula girl, in an effort to campaign for Turd Sandwich.