The Collaborative International Dictionary of English v.0.48:

Sound-board \Sound"-board`\, n.
   A sounding-board.
   [1913 Webster]

         To many a row of pipes the sound-board breathes.
                                                  --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soundboard
    n 1: (music) resonator consisting of a thin board whose
         vibrations reinforce the sound of the instrument [syn:
         sounding board, soundboard]