"Cantability" is a very rare word in English and almost always occurs as a calque in translated texts or those written by non-native speakers. "Cantabile" means "in a singing manner": it is an adjective.

The quality of vocal music that makes it easy or pleasant to sing is simply called "singability" by English-speaking musicologists:

"Then there is the singability of Dunstable’s melodies. They avoid difficult to sing jumps and instead move by step or alternating step with third to create an easily sung line with interesting patterns."
http://francisbarnhart.com/projects/john-dunstable/

"There is a possibility that Martin Luther himself might have modified the German text and that his general emphasis upon singability of the chants may have led others or someone else to come up with a 'melody' that would conform with the goals which Luther had been stressing."
http://www.bach-cantatas.com/CM/Magnificat.htm

"Renaissance ornamentation is vocal. Smoothness and singability are the norm. In the baroque period vocal and instrumental practices become separated."
http://www.americanrecorder.org/membership/pubs/freepubs/Con...

""Singing" translations (i.e., those translations often included under the original text in vocal scores) are generally worthless to an audience member who really wants to make sense of what's being sung. I try to find idiomatic English translations -- that is, translations that convey the sense of the original language without worrying about rhyme or singability."
http://facstaff.uww.edu/allsenj/MSO/NOTES/WritingNotes.htm

Many more examples could be quoted.

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Note added at 1 hr (2011-06-06 12:32:44 GMT)
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I should have said that "cantabile" is an adjective or adverb (usually the latter). It is a term almost always applied to instrumental playing that imitates singing: smooth, lyrical, expressive.