The Collaborative International Dictionary of English v.0.48:

Vocalize \Vo"cal*ize\ (v[=o]"kal*[imac]z), v. t. [imp. & p. p.
   Vocalized (v[=o]"kal*[imac]zd); p. pr. & vb. n.
   Vocalizing (v[=o]"kal*[imac]*z[i^]ng).] [Cf. F. vocaliser.]
   1. To form into voice; to make vocal or sonant; to give
      intonation or resonance to.
      [1913 Webster]

            It is one thing to give an impulse to breath alone,
            another thing to vocalize that breath. --Holder.
      [1913 Webster]

   2. To practice singing on the vowel sounds.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vocalizing
    n 1: the act of singing vocal music [syn: singing,
         vocalizing]