While participating in the The 4 Million Child Blow 2000 recorder concert, Eric Cartman and Kenny McCormick try to find the elusive brown note. The Brown Note is a certain pitch or frequency that makes people excrete feces.
It is stated in the episode that the French experimented with it during World War II. It is also stated that the Brown Note is believed to be 92 octaves below the lowest e flat. But during the episode, the Brown Note is discovered and used to make the whole world excrete feces.
The Brown Note would appear again as one of Jimmy's attacks in South Park: The Stick of Truth.