Here's a story about the first demonstration of spontaneous human voice mimicry in a cetacean (the collective term for whales and dolphins). Schevill and Lawrence who first studied the sounds of the white whale, or beluga, in the wild wrote that "occasionally the calls would suggest a crowd of children shouting in the distance". Listen to the recording in this article http://inkfish.fieldofscience.com/2012/10/you-have-to-hear-this-beluga-mimicking.html and make your own decision about what it sounds like.
These sounds were made by NOC, a beluga that lived at the National Marine Mammal Foundation in San Diego until he died five years ago. The whale was recognised as the source of the sound when one day a diver surfaced outside the whale's enclosure and asked "Who told me to get out?". Researchers have since done tests to understand how the sounds were made and more details can be found in a new paper in Current Biology http://www.sciencedirect.com/science/article/pii/S0960982212010093