The Collaborative International Dictionary of English v.0.48:

Sounding-board \Sound"ing-board`\, n.
   1. (Mus.) A thin board which propagates the sound in a piano,
      in a violin, and in some other musical instruments.
      [1913 Webster]

   2. A board or structure placed behind or over a pulpit or
      rostrum to give distinctness to a speaker's voice.
      [1913 Webster]

   3. pl. See Sound boarding, under Sound, a noise.
      [1913 Webster]