Source : Webster's Revised Unabridged Dictionary (1913)

Croon \Croon\, n.
   1. A low, continued moan; a murmur.

   2. A low singing; a plain, artless melody.

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

   2. To hum or sing in a low tone; to murmur softly.

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

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

            Hearing such stanzas crooned in her praise. --C.
                                                  Bront?.

   2. To soothe by singing softly.

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

Source : WordNet®

croon
     v : sing softly