The Collaborative International Dictionary of English v.0.48:

Sonant \So"nant\, a. [L. sonans, -antis, p. pr. of sonare to
   sound. See Sound a noise.]
   1. Of or pertaining to sound; sounding.
      [1913 Webster]

   2. (Phonetics) Uttered, as an element of speech, with tone or
      proper vocal sound, as distinguished from mere breath
      sound; intonated; voiced; vocal; tonic; the opposite of
      nonvocal, or surd; -- said of the vowels, semivowels,
      liquids, and nasals, and particularly of the consonants b,
      d, g hard, v, etc., as compared with their cognates p, t,
      k, f, etc., which are called nonvocal, surd, or aspirate.
      -- n. A sonant letter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sonant
    adj 1: produced with vibration of the vocal cords; "a frequently
           voiced opinion"; "voiced consonants such as `b' and `g'
           and `z'"; [syn: voiced, sonant, soft] [ant: hard,
           surd, unvoiced, voiceless]
    n 1: a speech sound accompanied by sound from the vocal cords
         [syn: sonant, voiced sound]