The Collaborative International Dictionary of English v.0.48:

Yodel \Yo"del\, Yodle \Yo"dle\, v. t. & i. [imp. & p. p.
   Yodeled, Yodled; p. pr. & vb. n. Yodeling, Yodling.]
   [G. jodeln.]
   To sing in a manner common among the Swiss and Tyrolese
   mountaineers, by suddenly changing from the head voice, or
   falsetto, to the chest voice, and the contrary; to warble.
   [1913 Webster] Yodel

WordNet (r) 3.0 (2006):

yodeling
    n 1: singing by changing back and forth between the chest voice
         and a falsetto