We have read in the Bible of a part where a donkey spoke to Balaam.

Is this actually possible? Yes going by science and what we have seen recently by some animals. It can't be ruled out that the donkey actually spoke.

Here are some animals scientifically proven through research that can speak or better still mimic the human speech or sound.

Orca whales
Research published last month proved that orca, or killer, whales have the ability to mimic the complexities of human speech. Josep Call, professor in evolutionary origins of mind at the University of St Andrews, was a co-author of the study. He said: “I think here we have the first evidence that killer whales may be learning sounds by vocal imitation.”

Rocky the ape
Dr Adriano Lameira of Durham University led a team of researchers in 2016 that proved orangutans have the ability to control their vocal range, communicating in a way not dissimilar to humans. The study centred on an ape named Rocky, from Indianapolis zoo, who mimicked the speech of his caretakers in exchange for food.

Koshik the elephant
Trainer Kim Jong-Gap gives a treat to Koshik, a 22-year-old male Asian elephant at the Everland Zoo in Yongin, south of Seoul on November 2, 2012.
Koshik, a male Asian elephant in a Korean zoo who can imitate human speech.
In 2012, Angela Stoeger-Horwath of the University of Vienna, captured footage of Koshik, a male elephant from Korea, communicating in a distinctly human way. Koshik would place his trunk into his mouth to create a vocal tract that facilitated the expression of pitches and formants similar to that of human speech.

Noc the beluga whale
Researchers at the National Marine Mammal Foundation in San Diego were taken aback in 1984 when they heard what sounded like people talking inside a whale tank. In fact, the human-like calls were coming from a captive male beluga whale named Noc. A study was subsequently conducted that proved the rhythms recorded from Noc’s wails were close in pattern and tonality to that of human speech.

Alex the parrot
Dr Irene Pepperberg, an animal psychologist from the University of Harvard, conducted a 30-year study of Alex, an African grey parrot. Before the research, it was widely assumed in the scientific community that since birds did not possess a primate brain, they would be incapable of performing complex mental tasks. Although Dr Pepperberg said that Alex wasn’t necessarily able to use language, she showed he could engage in two-way forms of communication and expression with humans. However recently we have seen a video of a cat about to be bathed in the tub making some sounds that sounded "Mary Mary I'm cold"

If it's possible for this animals to do that, the possibility of a donkey talking in the Bible should not be ruled out as well. Who knows may be the donkey mimicked some warning sounds in Hebrew to Balaam.
Link to the cat mimicking human speech
https://www.google.com/search?q=im+cold+im+cold+mari+mari+nooo&oq=Mari+Mari+I%27m+cold&aqs=chrome.2.69i57j0i13j0i8i13i30j0i390.11913j0j9&client=ms-android-xiaomi-rev1&sourceid=chrome-mobile&ie=UTF-8#fpstate=ive&vld=cid:e3711d34,vid:Z-qA1k2K8fg,st:0

Shalom