The Collaborative International Dictionary of English v.0.48:

Sounding \Sound"ing\, n.
   1. The act of one who, or that which, sounds (in any of the
      senses of the several verbs).
      [1913 Webster]

   2. (Naut.) [From Sound to fathom.]
      (a) measurement by sounding; also, the depth so
          ascertained.
      (b) Any place or part of the ocean, or other water, where
          a sounding line will reach the bottom; -- usually in
          the plural.
      (c) The sand, shells, or the like, that are brought up by
          the sounding lead when it has touched bottom.
          [1913 Webster]

   Sounding lead, the plummet at the end of a sounding line.


   Sounding line, a line having a plummet at the end, used in
      making soundings.

   Sounding post (Mus.), a small post in a violin,
      violoncello, or similar instrument, set under the bridge
      as a support, for propagating the sounds to the body of
      the instrument; -- called also sound post.

   Sounding rod (Naut.), a rod used to ascertain the depth of
      water in a ship's hold.

   In soundings, within the eighty-fathom line. --Ham. Nav.
      Encyc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sounding line
    n 1: (nautical) plumb line for determining depth [syn: lead
         line, sounding line]