WordNet (r) 3.0 (2006):

sound truck
    n 1: a truck equipped with a loudspeaker and used for
         advertising

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "sound truck":
   Gramophone, PA, PA system, Victrola, audio sound system,
   audiophile, binaural system, bitch box, bullhorn, cartridge,
   ceramic pickup, changer, crystal pickup,
   derived four-channel system, discrete four-channel system,
   four-channel stereo system, hi-fi, hi-fi fan, high-fidelity,
   intercom, intercommunication system, jukebox, magnetic pickup,
   monaural system, mono, needle, nickelodeon, phonograph,
   photoelectric pickup, pickup, public-address system,
   quadraphonic sound system, radio-phonograph combination,
   record changer, record player, sound reproduction system,
   squawk box, stereo, stylus, system, tape deck, tape recorder,
   tone arm, transcription turntable, turntable