Vocal processor

From Memory Alpha, the free Star Trek reference

A vocal processor was the speech-regulating subroutine of a hologram's matrix.

After the USS Voyager's holographic Doctor introduced her world to music in 2376, Qomarian composer Tincoo wrote a composition that was far beyond the ability of the Doctor's vocal processor to handle. (VOY: "Virtuoso")

When the Doctor continually berated Harry Kim for his piloting skills on an away mission in 2377, Seven of Nine jokingly suggested disconnecting his vocal processor for the remainder of the trip. (VOY: "Body and Soul")

Later that year, members of Iden's rebellion kidnapped Voyager Chief Engineer B'Elanna Torres to obtain her assistance with their photonic field generator. While testing the modifications, Torres transferred the holographic Cardassian Kejal to the generator, which initially caused a malfunction in Kejal's vocal processor. Torres was able to correct the problem. (VOY: "Flesh and Blood")