It's possible, they claim, that since Neandertal children grew to adulthood at a faster rate compared to modern human children, and since most language skills in modern humans are acquired during childhood, Neandertal children may not have had much time to develop language skills. Another interesting clue to Neandertal speech lay in the shape of the skull base. In modern human infants, the skull base is flat and the infant's larynx or voicebox is located high in the throat. As the child grows older, the larynx gradually descends down the throat, allowing for an increasingly wider range of sounds. At the same time, the shape of the skull base changes from flat to angled.
Neandertal skulls have a flat base.
The hyoid bone is an important anatomical feature for speech ability. The Neandertal hyoid bone was found to be similar to that of modern humans, not ape-like as some had speculated.