The vocal subroutine was a computer program integrated into Emergency Medical Holographic programs.
While in sickbay aboard the USS Prometheus in 2374, The Doctor asked the EMH Mark II to disengage his vocal subroutines for one second so he could explain why he was aboard the Prometheus. (VOY: "Message in a Bottle")
In 2376, the Qomar Abarca suggested to his associate Tincoo to deactivate The Doctor's vocal subroutine and to re-program him but forgot this suggestion when he heard The Doctor sing. (VOY: "Virtuoso")