Later... (added on Oct 9, 2010):
Essentially, in apes the larynx is higher and the epiglottis can lock with the velum; in humans the larynx is too low for this to happen. Also, ape tongue movement is mostly in-out, while humans can move the muscle up and down as well as in-out. Furthermore, the tube through which air passes from the glottis out to the lips is gently curved in apes, but in humans it forms a right angle. Anyone who plays any kind of wind instrument knows that different shapes produce different sounds.
What all this means is that apes (and human newborns, who are similar) cannot produce sounds with the acoustic properties of adult human speech. And it's why it was such a stroke of genius to try out manually-produced sign languages on them.
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