Lincoln's voice is found in the Museum of History offices. It is located in the library (the 2-story room with the turret), upstairs, on a desk to the left of the hole blown in the main room up there.
The object itself is a phonautograph, the earliest known recording device invented by Frenchman Édouard-Léon Scott de Martinville in 1857. Unlike Edison's phonograph invented years later in 1877, the phonautograph was only designed to transcribe sound waves for visual analysis, and incapable of playing back a recording.
Lincoln's Profit Margins - Hannibal Hamlin or Leroy Walker will give the player character 50 caps for the device, or 100 caps with a successful Speech check. Also, Abraham Washington will pay 60 caps, or 120 after a successful Speech check.
Though it no longer serves any purpose, this item is still found in the game files for Fallout: New Vegas.
Behind the scenesEdit
This is a reference to a legend that such a recording was actually made of Lincoln's voice in 1863 by Édouard-Léon Scott de Martinville, the inventor of the phonautograph. Since the story did not appear until the 1960s, its authenticity is questionable.