What sound inventory could be pronounced by aliens without human mouths?
I think that the answer to this question is going to depend on what kinds of speech production organs the aliens do have.
An alien vocal tract might include nostrils that could be opened and closed to make plosives and fricatives; teeth-like projections on the tongue and tough ridges on the surrounding cavity that could facilitate non-human taps and clicks; the use of more than one valve in the vocal tract to produce staggered or simultaneous glottal noises....I'm sure that an anatomist with a science-fiction writer's imagination could come up with a longer list.
It's important to remember that aliens might not use their mouths for speech. The airway and the digestive tract might not intersect in an alien species. What is more, strictly speaking, humans don't have any speech organs. Everything we use in order to produce speech has some other, more important function, such as eating or protecting the airway. Our vocal tract is *adapted* for speech, but an alien's body might have some other body parts adapted for communication.
Since you're talking about making a human sound inventory that aliens without human mouths could produce, I assume that
you're ruling out the idea that the alien speech production organs might be like a parrot's, which can imitate all kinds of human speech, or even better, like the membrane on a loudspeaker, which might produce almost any sound at all.
In short, I think you should decide what kind of speech production organs your aliens have before you come up with your inventory.
I'm working with the equivalent to weres (not full transformation, more humanoid...and humanoid is so ugly a word in this case, but it's probably the best way to say not Twilight or American Werewolf in London weres :P), so they can pronounce pretty similar sounds. But having muzzles does have a marked impact, especially since even with lips, they can't round them the same way, and they would be positioned differently to a Human's. I have even chucked out labio-dentals, although having the labio-dental nasal as a phoneme would be fun (no recorded language appears to have it as a phoneme, only an allophone).
Erm...maybe my post and Rainchild's could be put into the non-Human mouths thread? I don't wish to derail this one. And my other post might be best in the Q & A thread, come to think of it...