WordNet (r) 3.0 (2006):

scat singing
    n 1: singing jazz; the singer substitutes nonsense syllables for
         the words of the song and tries to sound like a musical
         instrument [syn: scat, scat singing]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "scat singing":
   bel canto, bravura, choral singing, coloratura, croon, crooning,
   folk singing, hum, humming, intonation, lyricism, operatic singing,
   scat, singing, sol-fa, sol-fa exercise, solfeggio, solmization,
   song, tonic sol-fa, vocal music, vocalization, warbling, yodel,
   yodeling