The Collaborative International Dictionary of English v.0.48:

Croon \Croon\ (kr[=oo]n), v. i. [OE. croinen, cf. D. kreunen to
   moan. [root]24.]
   1. To make a continuous hollow moan, as cattle do when in
      pain. [Scot.] --Jamieson.
      [1913 Webster]

   2. To hum or sing in a low tone; to murmur softly.
      [1913 Webster]

            Here an old grandmother was crooning over a sick
            child, and rocking it to and fro.     --Dickens.
      [1913 Webster]

   3. To sing in a soft, evenly modulated manner adapted to
      amplifying systems, especially to sing in such a way with
      exaggerated sentimentality. --MW10 --RHUD
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Croon \Croon\, v. t. [imp. & p. p. Crooned (kr[=oo]nd); p. pr.
   & vb. n. Crooning.]
   1. To sing in a low tone, as if to one's self; to hum.
      [1913 Webster]

            Hearing such stanzas crooned in her praise. --C.
                                                  Bront['e].
      [1913 Webster]

   2. To soothe by singing softly.
      [1913 Webster]

            The fragment of the childish hymn with which he sung
            and crooned himself asleep.           --Dickens.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Croon \Croon\, n.
   1. A low, continued moan; a murmur.
      [1913 Webster]

   2. A low singing; a plain, artless melody.
      [1913 Webster]

WordNet (r) 3.0 (2006):

croon
    v 1: sing softly

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "croon":
   anthem, ballad, carol, chant, chirp, chirrup, choir, chorus,
   descant, do-re-mi, hum, hymn, intonate, intone, lilt, minstrel,
   pipe, psalm, quaver, roulade, serenade, shake, sing,
   sing in chorus, sol-fa, solmizate, tremolo, trill, troll, tweedle,
   tweedledee, twit, twitter, vocalize, warble, whistle, yodel