i Register
In some senses, mouthpiece is marked as slang. Watch for register when choosing this word.
ADJ
phone/telephone
VERB + MOUTHPIECE
cover (up), put your hand over
She covered the mouthpiece while she whispered something to her colleague in the office.
ADJ
official, political
VERB + MOUTHPIECE
act as, be
PREP
~ for/of
The spokesperson became the mouthpiece of the environmental organization after the director retired.