the.com/mouthpiece

means A person, publication, or device that speaks or amplifies views on behalf of someone else, or the part of an instrument/phone held to the mouth.

from Straightforward English compound — mouth plus piece — appearing in the sense of 'part of an instrument put to the mouth' before stretching, by the 19th century, into slang for a lawyer or spokesperson who 'does the talking' for someone else. The jump makes sense: just as a trumpet's mouthpiece is the part through which breath becomes sound, a human mouthpiece is the part through which someone else's will becomes words.