The Collaborative International Dictionary of English v.0.48:

Sound \Sound\, n. [AS. sund a narrow sea or strait; akin to
   Icel., Sw., Dan. & G. sund, probably so named because it
   could be swum across. See Swim.] (Geog.)
   A narrow passage of water, or a strait between the mainland
   and an island; also, a strait connecting two seas, or
   connecting a sea or lake with the ocean; as, the Sound
   between the Baltic and the german Ocean; Long Island Sound.
   [1913 Webster]

         The Sound of Denmark, where ships pay toll. --Camden.
   [1913 Webster]

   Sound dues, tolls formerly imposed by Denmark on vessels
      passing through the Baltic Sound.
      [1913 Webster]