mouthpiece

Short definition from Wiktionary

Noun

1. 1
   A part of any device that functions in or near the mouth, especially: The part of a telephone that is held close to the mouth.
2. 2
   A part of any device that functions in or near the mouth, especially: The part of a wind instrument that is held in or against the mouth.
3. 3
   A mouthguard.
4. 4
   A spokesman; one who speaks on behalf of someone else.
5. 5
   A lawyer for the defense.