From 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]