Of course, just pushing an "aaaaaaah" sound out of your throat is very limited as far as communication goes. To be able to make sounds that others will understand, we "articulate" speech, which involves precise movements of our tongue and jaws and lips in complicated ways. Have a look inside this speaker's head and you will see the intricate dance our throat and mouth perform when we speak:
You even move a piece of your anatomy you may not even know you have, such as the "soft palate" or velum, which sits at the back of the roof of your mouth!
How sensible do you think it is to expect deaf people to "lip read", if so many of the movements that shape speech sounds happen deep inside the speaker's head, and are normally hidden from view?