Sound requires something to creat a vibration and a soundbox to amplify that vibration to an audible level. Therefore in a fiddle the strings vibrate and the soundbox(Body of the fiddle amplifies the sound. The length and thickness of the strings determine the vibration and the shape and size of the soundbox effect the amplification and to a degree the tone of the produced sound, A fiddle may be roughly the same shape as a Cello but the sound is vastly different.

In the human instrument i.e. voice the vibrations are produced by the the two membranes of the voicebox and the amplification and tone by the oral cavity. Therefore the shape and bone structure of any human or particular group of humans will create slightly different sound patterns.