The elasticity in the balloon is acting on the air inside and forces it out the opening when you release the throat of the neck of the balloon. In case you are interested, if you do not totally release the balloon and instead hold on to the intake port and allow the air to escape in a whine or whistle mode, you are sensing the sonic effects of the escaping air. In certain cases of fluid flow through a nozzle, you actually achieve the speed of sound, which is why you hear some of the things you do. I cannot make it simpler than that. If, however, you seek to understand the principle a little further, just pick up a basic college book on fluid dynamics, and it will explain things a bit more.
Science can actually be fun, despite the complicated math, etc.
It always was to me, at least. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.