I have university degrees in both cognitive neuroscience and linguistics, so I feel like I should absolutely know this. But somehow, I only have an average person's sense of it. Blame my professors I guess.
I was taught that the way the vocal tract is organized at birth is not well adapted for speech, and later development is required to enable the full range of possible phonemes. But obviously, newborn babies can vocalize, and have at least some range of tongue movement. So imagine a Mushoku Tensei-esque scenario; an adult mind is put into a newborn, and they really want to say something. What types of phonemes does the structure of their vocal tract, combined with the motor functionalities of their brain, allow them to produce at that age?
We all know that the /a/~/æ/ spectrum is one of them, certainly /ə/ also, and vowels in general are probably the most likely candidates. Given our hypothetical subject's foreknowledge and intentionality, could we add a few consonants to their repertoire as well? Labials would be my first guess, as they're often the first consonants to develop in L1 learners, and lips aren't a part of the apparatus that really needs any postnatal development. Alveolars might not be far behind? But I'm just guessing. I'm sure there is some hard data out there relevant to this question, some study analyzing neonatal vocalizations for phonemic diversity, but I'm not familiar with any of it. I'd really like to read a paper like that.