(written from a Production point of view)
Referenced in the final revised draft of the script for the Star Trek: The Original Series episode "The Conscience of the King", a voice track was a way to graph a person's voice. It could be displayed on a piece of paper.
In the script, Captain Kirk was about to show Spock a slip of paper that showed a voice track of Kodos, while trying to ascertain whether Kodos was actually now Anton Karidian, when the noise of a phaser on overload suddenly interrupted Kirk and Spock. In the final version of the episode, however, Kirk simply begins to say that he has an idea which may prove Karidian innocent, though Kirk is abruptly silenced by Spock.