"Jarabe Tapatío", also known as "The Mexican Hat Dance", is an extremely popular Mexican dance. It was featured prominently in the Season Seven episode, "Casa Bonita". As heard on South Park, it contains no lyrics.
The music is first played in "Fun with Veal" during the Puppies from Around the World video montage.
In "The Last of the Meheecans", when it is announced on the radio that Butters Stotch had crossed the border into the United States, the Mexicans begin celebrating with the music playing in the background.