scat singing

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music, jazz The act of vocalizing, using nonsense syllables or sounds (e.g. "dool-yuh doot-n dwee-dah") to create an improvised melodic solo, often imitative of other musical instruments.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun singing jazz; the singer substitutes nonsense syllables for the words of the song and tries to sound like a musical instrument