The Threepenny Opera ends with a sound: "The bells of Westminster are heard ringing for the third time" (3.9.375). Westminster Abbey's bells would have been ringing to celebrate Queen Victoria's coronation.
It could be that Brecht chose to end the play with this sound because it will remind the audience of the power behind all of the greed, hypocrisy, and general banditry presented in the play. The madness is a result of the political structure, in which some people have absolute power and others have none.