By one account, the Voice Integrator was the part of a TARDIS which helped travellers understand foreign languages. It automatically detected the language of a planet and used the telepathic circuits to alter the perceptions of both the travellers and the locals, such that both perceived the other to be speaking their language. For the Doctor, this resulted in English being spoken wherever he went.
The Voice Integrator was invented by Therde, who used theories from the Doctor as the basis for its functioning.
On the TARDIS Console used by the First Doctor, the Voice Integrator was located on the third panel from the doors. (PROSE: TARDIS Console [+]Loading...["TARDIS Console (DWM 79 feature)"])