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]

Moby Thesaurus II by Grady Ward, 1.0:

155 Moby Thesaurus words for "mouthpiece":
   AG, DA, KC, QC, SSC, US attorney, adviser, advocate, aerophone,
   agent, amicus curiae, announcer, annunciator, attorney,
   attorney general, attorney-at-law, authority, barrister,
   barrister-at-law, bell, bit, call box, channel, civilian,
   coin telephone, communicant, communicator, connection,
   corporation lawyer, counsel, counselor, counselor-at-law,
   court-appointed lawyer, criminal lawyer, defense counsel, delegate,
   deputy, desk telephone, dial telephone, district attorney,
   double reed, embouchure, enlightener, expert witness, extension,
   friend at court, front, front man, go-between, gossipmonger,
   grapevine, herald, horn, informant, information center,
   information medium, informer, interagent, interceder, intercessor,
   intermediary, intermediate, intermediate agent, intermediator,
   intermedium, internuncio, intervener, interventionist, interventor,
   interviewee, junior counsel, key, law agent, lawyer, leader,
   legal adviser, legal counselor, legal expert, legal practitioner,
   legalist, lip, mediator, medium, middleman, monitor, mouth,
   negotiant, negotiator, negotiatress, negotiatrix, newsmonger,
   notifier, official spokesman, ombudsman, pay station, phone, pipe,
   pleader, press, private attorney, proctor, procurator, prolocutor,
   prolocutress, prolocutrix, prosecuting attorney, prosecutor,
   public prosecutor, public relations officer, public telephone,
   publicist, publisher, push-button telephone, radio, rapporteur,
   receiver, reed, reporter, representative, sea lawyer,
   self-styled lawyer, shyster, silk, silk gown, slide, solicitor,
   solicitor general, source, speaker, special pleader, spokesman,
   spokesperson, spokeswoman, stuff gown, stuff-gownsman, telephone,
   telephone booth, telephone engineering, telephone extension,
   telephone mechanics, telephone receiver, telephonics, telephony,
   television, teller, tipster, tooter, tout, transmitter, valve,
   voice, wall telephone, wind, wind instrument, witness