The Collaborative International Dictionary of English v.0.48:

Mouthpiece \Mouth"piece`\, n.
   1. The part of a musical or other instrument to which the
      mouth is applied in using it; as, the mouthpiece of a
      bugle, or of a tobacco pipe.
      [1913 Webster]

   2. An appendage to an inlet or outlet opening of a pipe or
      vessel, to direct or facilitate the inflow or outflow of a
      fluid.
      [1913 Webster]

   3. One who delivers the opinion of others or of another; a
      spokesman; as, the mouthpiece of his party.
      [1913 Webster]

            Egmont was imprudent enough to make himself the
            mouthpiece of their remonstrance.     --Motley.
      [1913 Webster]

   4. Hence: A person's lawyer. [slang]

   Note: This is a term that was used sometimes in old movies.
         When a tough bad guy was arrested he might say "I ain't
         sayin' nothin' without my mouthpiece!"
         [PJC]

WordNet (r) 3.0 (2006):

mouthpiece
    n 1: a part that goes over or into the mouth of a person; "the
         mouthpiece of a respirator"
    2: an acoustic device; the part of a telephone into which a
       person speaks
    3: a spokesperson (as a lawyer) [syn: mouthpiece, mouth]
    4: (especially boxing) equipment that protects an athlete's
       mouth [syn: mouthpiece, gumshield]
    5: the tube of a pipe or cigarette holder that a smoker holds in
       the mouth
    6: the aperture of a wind instrument into which the player blows
       directly [syn: mouthpiece, embouchure]