Monkeys could talk, if they only had the right brain circuitry.
An influential 1969 paper examined the cadaver of a rhesus macaque (Macaca mulatta) and concluded that its vocal anatomy was not capable of speech. To reassess this claim, a team led by Tecumseh Fitch at the University of Vienna and Asif Ghazanfar at Princeton University in New Jersey X-rayed live long-tailed macaques (Macaca fascicularis, pictured) as they made various sounds, such as threat calls. Using the scans, the authors developed a computer model of the macaque vocal tract. This suggested that the monkeys do have the anatomy to make speech sounds, including five vowels and even the phrase “Will you marry me?”
Monkeys can't speak because they lack the brain circuitry required for fine motor control, vocal learning and other attributes necessary for speech, the authors say.