mouth ‧ piece /ˈmaʊθpiːs/ BrE AmE noun [countable]
1 . the 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] a person, newspaper etc that expresses the opinions of a government or a political organization
mouthpiece of
The newspaper was the mouthpiece of the National Democratic Party.