Seeing the movie as a kid I too thought the sound was the sand people and not a Jedi making the call of a "never actually seen or heard in any Star Wars ever" beast (you do fight one in KOTOR though). I feel that living there for so long, he would have some sort of agreement, or history with them to warrant such a response.
The need for him to make that call every time he wants a sand peoples to flee would start a "cried wolf" effect to which they would learn anyway that they are encountering something more scary than a Krayt Dragon, an exiled Jedi. which goes back to point one, why front as a Krayt Dragon if you can pull the Jedi card and cause very loud frightening screams followed by top speed fleeing by sand people all the time. Seems weird to me. Pun intended.