Mouthpiece

wiktionary

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mouth +‎  piece

etymonline

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mouthpiece (n.)

also mouth-piece, 1680s, "casting fitted on an open end of a pipe, etc.," from mouth (n.) + piece (n.1). Meaning "piece of a musical instrument that goes in the mouth" is from 1776. Sense of "one who speaks on behalf of others" is from 1805; in the specific sense of "lawyer" it is attested by 1857.