Birds don't have vocal cords, what they do have is a syrinx, which is more drum-like to the human vibrating strings.
So when a bird mimics human speech, it's less repeating the words it hears, so much as replicating the noises in a human "song". The syrinx is the main reason birds have such a wide vocal range (I'd like to hear you perfectly replicate the sound of plastic clattering inside a vacuum).
Humans rely on our lips, tongues, and mouths to provide the diversity of sound required for our complex verbal languages (birds rely on the "drum" in their neck). However for a group of the stiff-beaked races above, any sounds requiring motion beyond opening/closing your mouth should be discounted if relying on human vocal cords. Pull back your lips(with the attached muscles, not your fingers) and try to pronounce the sound. For instance, "Oh" requires lip movement, while "Ah" and "A" do not.
- There are 44 "sounds" in the English language.
- here you can find a downloadable chart for a more international scale.
But you can bypass this entire problem by giving the bird-people syrinxes.
 thank you to @John for allowing me to realize my assumption of preferred language, it was narrow-minded of me and I apologize.