In my world, through the work of magic, a group of humans were mutated into half-bird people. I've figured out most of the anatomy so I wanted to know how their speech would work. If it matters, before the mutation they had a culture inspired by a mix of Chinese and Brazilian. They have the throat tissues of a human, but basically the skull of a bird. I know birds don't have lips so P's and B's are out, and their tongues aren't as flexible so they can't do sounds like sh and ch, or French R's, but do you think they could get away with pallet and behind-the-teeth sounds? They obviously don't have teeth but could their beak act like one? Alternatively how do you think the Mandarin and Portuguese languages would evolve with these restraints? I'm curious to hear your input and samples or examples of your ideas would be cool to see!
You don't need to worry about what sounds we can make that they can't. Unlike humans, birds don't modulate the sounds they vocalize on the mouth. They have an organ in the chest for that, the avian syrinx. Give them this organ and you have one less thing to worry about.
The syrinx (Greek σύριγξ for pan pipes) is the vocal organ of birds. Located at the base of a bird's trachea, it produces sounds without the vocal folds of mammals. The sound is produced by vibrations of some or all of the membrana tympaniformis (the walls of the syrinx) and the pessulus, caused by air flowing through the syrinx. This sets up a self-oscillating system that modulates the airflow creating the sound. The muscles modulate the sound shape by changing the tension of the membranes and the bronchial openings. The syrinx enables some species of birds (such as parrots, crows, and mynas) to mimic human speech.
Don't believe me just because I'm quoting Wikipedia. Check this video of a cute african grey talking.
-What's your name?
-Petra, where's your laser?
-pew pew pew pew pew!
She even does peek-a-boo's!
If you don't want them to have a syrinx, then yeah remove the P and M sound from their language. They'll never be able to pronounce the B sound, but they might me able to to the V sound from Spanish, which is basically the same as B for people with Que Hora Es level of Spanish.
I would put more thought into sounds that they can do and we can't. Avian people could probably add many whistles, clicks and Star Wars droid sounds as phonemes to their language which would be hard for people to precisely mimic.