A subvocal processor was a speech-regulating device implanted in some Borg drones.
When the USS Voyager first encountered the adolescent Borg Icheb in 2376, his subvocal processor was malfunctioning, causing his speech to sound hollow and partially garbled. Seven of Nine made a minor adjustment to correct the problem, despite First having been told by their leader that the malfunction could not be repaired until he rejoined the Borg Collective. (VOY: "Collective")
In 2377, Voyager Chief Engineer B'Elanna Torres was fitted with a subvocal processor after she, Tuvok and Captain Kathryn Janeway were assimilated by the Borg during a covert mission to aid the inhabitants of Unimatrix Zero. Torres' voice took on a hollow sound, alerting Tuvok to the fact that the processor had been implanted. Torres said she could live with it. (VOY: "Unimatrix Zero, Part II")