Gorillas have been taught to use sign language. Chimpanzees understand a symbolic language known as 'Yerkish' to communicate with humans. Yet the holy grail of communicating with our primate cousins is speech. While monkeys and apes are capable of conveying information with vocalizations, so far humans haven't gotten them to say something as simple as "No" under any circumstances.
Until recently there were two schools of thought regarding their vocal deficiency. The first is the neural hypothesis favored by Charles Darwin. In short, primates have all the necessary equipment to be able orators, except the wiring to coordinate and control their vocal apparatus is lacking. On the other hand, Philip H. Lieberman, and a few friends, put the kibosh on this hypothesis using plaster casts of a dead rhesus monkey's vocal tract. In their 1969 paper, Vocal Tract Limitations on the Vowel Repertoires of Rhesus Monkey and other Nonhuman Primates', they concluded monkeys weren't capable of enunciating the vast array of sounds particular to humans.
A new paper submitted to Science Advances shows the pendulum swinging back in the other direction. Researchers at Princeton University and the University of Vienna have proven monkey vocal tracts are speech-ready. Rather than using a cadaver, William Tecumseh Fitch and Asif Ghazanfar taught a macaque to sit in a chair while they took x-ray videos of the monkey's throat. Once they had enough footage showing most configurations of their vocal tract, the data was plugged into a program capable of simulating speech with surprising results.
They chose the question, "Will you marry me?" because it contains all five vowels of the English language. If you don't hear it, compare it against a human voice simulated with the same software asking the same age-old question.
Which raises the question, "What is it about the human brain that makes it so special?" Conventional science believes the interaction between the FOXP2 and Srpx2 genes might be a factor. Monkeys have these genes, but they're different from ours by two base pairs. Perhaps sometime in the near future, scientists will CRISPR in human analogues of the genes to open a long-overdue interspecies dialogue.