From Longman Dictionary of Contemporary Englishmouthpiecemouth‧piece /ˈmaʊθpiːs/ noun [countable] 1 
APMTCTthe part of a musical instrument, telephone etc that you put in your mouth or next to your mouth Ben put his hand over the mouthpiece and shouted to me.2 [usually singular]PGREPRESENT a person, newspaper etc that expresses the opinions of a government or a political organizationmouthpiece of The newspaper was the mouthpiece of the National Democratic Party.
APMTCTthe part of a musical instrument, telephone etc that you put in your mouth or next to your mouth Ben put his hand over the mouthpiece and shouted to me.2 [usually singular]PGREPRESENT a person, newspaper etc that expresses the opinions of a government or a political organizationmouthpiece of The newspaper was the mouthpiece of the National Democratic Party.Examples from the Corpus
mouthpiece• The Churches have a clear responsibility to be a mouthpiece for those who don't have a platform.• Jerry painted with a brush held by a mouthpiece.• The same held true for mouthpieces for wind instruments and replacement roots for teeth, Sakai explained.• In 1917 Stalin became editor of Pravda, the official mouthpiece of the Communist Party.• Pidgeon's bizarrely flat mannerisms and speaking style make her the perfect mouthpiece for Mamet's best lines.• The Chemical Manufacturers Association is the mouthpiece of the American chemicals industry.• Clayt lifted the mouthpiece to the radio.• It was the same old-fashioned type as Miss Tuckey's and he unscrewed the mouthpiece carefully but found nothing extra inside.• He unscrewed the mouthpiece which had been hastily jammed on and caught by the thread.• Pravda formally was the mouthpiece of the Communist Party.• These plays were the mouthpiece of the revolutionary bourgeoisie and always reflected their values. 