I'm not so sure about the "voice of God" framing, but there are several accounts of experiments to determine the original language of humanity by isolating infants and seeing what language they spoke naturally.

The earliest such account of which I'm aware is found in Herodotus' Histories. In the story, which Herodotus would presumably have heard from local sources during his research trip to Egypt, the Pharaoh Psammetichus (i.e., Psamtik I, r. 664-10 BCE) ordered that two newborns be given to a goatherd, who was to raise them in isolation and never speak to them. After two years, the goatherd reported that one of the infants had said "bekos." Psammetichus learned that this sounded like the Phrygian word for bread, so he concluded that the Phrygians (not the Egyptians) were the oldest people.

Subsequently the 13th century chronicler Salimbene wrote that the cosmopolitan Emperor Frederick II (r. 1220-50) conducted a similar experiment by ordering women to raise babies without talking to them. This resulted in the deaths of the babies, who " could not live without clappings of the hands, and gestures, and gladness of countenance, and blandishments." Salimbene specifically compared this alleged experiment to that reported by Herodotus. He included it in a list of vain, gruesome experiments allegedly conducted by Frederick.

Chronicler Robert Lindsay of Piscottie credited James IV of Scotland (r. 1488-1513) with conducting a similar experiment by having a mute nurse raise two babies on an small island. Lindsay reported that some people said the children spoke good Hebrew (a perennial candidate among medieval Christians for "world's oldest language") but he did not know the truth of this himself. Sir Walter Scott, compiling his own history of Scotland centuries later, suggested, "It is more likely they would scream like their dumb nurse, or bleat like the goats and sheep on the island."

It seems likely none of these experiments actually happened. The medieval accounts were colored by knowledge of Herodotus. It became something of a trope that a ruler with nothing better to do would try to determine the true original language through a language deprivation experiment.

Bookending this somewhat, the Enlightenment philosopher Montesquieu (1689-1755) suggested that some wise and inquisitive rule ought to isolate some children and see what kind of language they develop and otherwise use the experiment to determine what sort of society is natural. This overlapped somewhat with the idea of feral children (i.e., children growing up without human contact due to misfortune and "raised by wolves" or something like that) who were also thought to give insight into the natural state of humanity. The idea of a conscious experiment was novelized during this period by Gaspard Guillard de Beaurieu, L'eleve de la nature (1761), later made into a successful melodrama.

In none of the accounts I've read was the language the children would spontaneously speak credited to "speaking to God." Rather it would just be some kind of natural phenomenon.

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