Why doesn't steady flow of air create sound? When one exhales air by twisting the lips appropriately, one does not create enough sound. But when the same amount of air with the same pressure is blown in a flute or whistle, a relatively audible sound is produced.
Is it because in a flute or whistle the air is used to vibrate the air inside the instrument and when blowing directly into air the pressure of the exhaled air doesn't create any vibrations?
 A: Steady laminar flow between two fluid layers is often inherently unstable [Rayleigh 1894, The Theory of Sound].  When the boundary is sharp and the difference in speed is significant, this instability can create audible noise.  This is the core mechanism of sound production in both whistling with the lips and some instruments like the flute and flue organ pipes.
The other feature of these sounds is resonance from a cavity and sometimes a feedback of the resonance to the driving oscillation.  In instruments, the feedback is enhanced by a sharp mechanical edge in the flow stream (an edge in the mechanical windway in pipes and recorders, and the opposite edge of the mouthpiece hole in a flute), so that the oscillation alternates to opposing sides of the edge.
It's interesting that in some configurations of whistling one can transition from no sound to sound by: 1) blowing harder (enhancing the speed difference between the outgoing and stationary air); or 2) introducing a knife edge perpendicular to the airflow (reducing the width of the boundary between the two flow regions).
To answer your exact question: steady flow in air can produce a sound.  Whether it does depends on the boundary between the flowing and stationary air (and because it's unstable, whether you now call this "steady" is a bit a question of semantics).
A: You are in fact asking why a sound is produced by a specific musical instrument but not when you blow air with your mouth.
Sound is produced by vibrations in a media, here air. When you blow no vibrations ( obviously there are some that cause a little sound of blowing to be heard) are caused as for you to here a sound. Instead, a musical instrument is build with such a geometry as for sound to be produced; even more sound that we may call music.
I hope this covers your question. You can dig up a lot of thinks in the web for musical instruments and how they work and there also exist many books on the subject.
