Mouthpiece (noun)

1. A part of a musical instrument, such as a trumpet or saxophone, that is placed in the mouth and played by blowing into it.
2. A person who speaks on behalf of or represents another person or group.
3. A device, such as a mask or respirator, that covers the mouth and nose and is used to filter or purify the air breathed in.

Origin:

From mouth + piece, first attested in 1834.

Examples:

1. He carefully inserted the mouthpiece of his saxophone and began to play.
2. The spokesperson is the official mouthpiece of the company.
3. The doctor gave her a mouthpiece to wear during her sleep to help her breathing.
4. The politician used his campaign manager as a mouthpiece to communicate his ideas to the public.
5. The lawyer acted as the mouthpiece for his client in court.