How does a steady wind flow generate sound? When a wind blow through sharp edge, say, edge of a paper, you can see the vibration of the paper and hear the sound.
For this type of oscillation, it should be a damped oscillation with external driving force $\mathcal{F}(x)$:
$\ddot{x}+2\lambda\dot{x}+\omega_{0}^{2}x=\mathcal{F}(x,t)$
with the driving force in the form of sinusoidal $\mathcal{F}(x,t)\sim F\sin(\omega t)$ (It should be a 3D version though). However, if the air flow is really steady, the external force should be constant so there is no oscillation at all from this equation.
A simple guess is that the motion of the paper influence the streamline of air flow so that the pressure gradient provides the exact sinusoidal driving force. This mechanism is reasonable because the propagation of sound wave has the oscillation relationship between the displacement and the pressure. A question here is how the air flow sustain the energy loss due to the damping.
It is also not easy to think of the initial condition as well as external force for these oscillation. So 
1) What exactly is the physical mechanism to generate sound when a 'steady' air flowing through a sharp edge?
2) Another similar question is how the driving force act in the resonance in musical instrumental such as pipe? A 'steady' flow of air is also provided somewhere to the pipe, but air oscillates inside the pipe.
Edit: From the answers, people suggest few mechanism for different situations. Few more questions:
3) For static object, Kármán vortex street can form behind the object. So is the sound frequency the same as frequency of vortex generation, or the same as the expansion of the vortex? Sound is a spherical wave propagating outward, so identify the sound frequency should locate the point of generation.
4) Where is sound generated in the situation (1)? Is the sound frequency the same as the frequency of the vibrating paper?
Fluid mechanics is a difficult topics, but there are approximation for special cases trying to explain the underlying mechanism, such as the flag fluttering cited by j.c.
 A: The full description of viscous fluid flow (i.e. the Navier-Stokes equations) is non-linear and can be sensitively dependent on initial conditions. What this means in practical terms it that you can't always count on your intuition.
The evolution of the Kármán vortex streets linked by mbq from laminar flow around an obstacle are are a classic demonstration. (And easy enough for a determined middle school student to create in the garage for a science project, though things are likely to get wet...)
Any way, once the wind starts doing non-linear things, it can generate periodic stresses, and from that you get the whistling or humming noise we all know and love. Add a resonant cavity and you can amplify nice pure tones which is the short-short version of how this works in wind instruments.
A: There are really two parts to your question.  First, how does the wind affect the motion of the paper, and how does that motion then couple to the behavior of the wind?
For the case of a flag fluttering due to the wind (which may or may not be applicable depending on what you have in mind), the physics of the instability leading to fluttering has been worked out in a fairly celebrated paper of 2005 by Argentina and Mahadevan.  They argue that:

[...] in a particular limit corresponding to a low-density fluid flowing over a soft high-density flag, the flapping instability is akin to a resonance between the mode of oscillation of a rigid pivoted airfoil in a flow and a hinged-free elastic plate vibrating in its lowest mode. 

I suspect this paper and probably some of the papers that cite it would be the right place to look.  As you can see, this part of the question that you've asked is of fairly high interest in current research.
Second, how does this motion generate sound?  This is also an interesting question but I know less about this.  I would suppose that some resonant vibrational modes of the sheet of paper that you're asking about (those involved in the flapping modes described by Argentina and Mahadevan) generate the sound.  But I don't know very much about sound generation or acoustics.
A: It is always about a resonance between the vibrating element and a pressure mod in some resonator (in case of sharp edge, the edge itself oscillates).
I believe the oscillations itself are produced by the turbulent vortices forming Kármán street behind the reed or edge.
EDIT: I have even found a reference: http://arxiv.org/abs/physics/0008053v1 
EDIT2: In detail, the process starts with static edge in laminar flow; speed of flow increases and finally the Reynolds number exceeds critical value in which streamlines start to disengage from edge first forming vortices in Karman street (this is called Strouhal instability) and then a totally random turbulent flow. The vortices interact with the edge "poking" it quite randomly (Karman street has some frequency, but I think it does matter only for some narrow cases); now the process is driven by resonances. The edge has some resonance frequencies, the air around can also have them if enclosed in some container, in case of some instruments the musician's lips are also involved -- all those for some kind of mechanical filter that amplifies certain frequencies (instruments) or ranges of frequencies (random setups). Finally, those mechanic vibrations induce pressure weaves in air that we hear as a sound, again either clear as in case of instruments or a noise as in case of trees, roofs and other stuff.
A: This is a similar problem to the karman vortices developing around cables/pipes when wind blows around them. When each vortex sheds and releases from the boundary layer there is a reactive aerodynamic "lift" developed which might move the structure. When the structure returns it forces the next vortex to shed aplifying the effect. It is like pushing a pendulum. If you push on the right time, even a tiny force can have a large effect.
Search cable galloping, aeolian vibrations and structural wind noise. Note that with cables the wind might excite the 160-th basic harmonic of the system, and not the first few as you might expect. Thats because the internal turbulence in wind resonates based on the Strouhal number is it might be of the order of 40 Hz-160 Hz.
A: How does a steady flow generate sound?
Airflow along a smooth boundary creates long-crested simple harmonic (SH) boundary layer oscillations (Tollmien-Schlichting shear waves, or T-S waves) as proven by Schubauer and Skramstad and Skramstad, 1941). What has been overlooked over the years is that an oscillation of a mass in a fluid creates a sound wave and, therefore, simple harmonic oscillations (fluttering) of a mass of fluid,
flowing in layers along a boundary, create simple harmonic sound. Not surprisingly, Schubauer and Skramstad inserted a ferromagnetic ribbon into the boundary layer fluid, showing that electromagnetically-induced SH vibrations of the ribbon could create, amplify or damp, the T-S waves that precede the onset of turbulence. As these fluttering boundary layer oscillations slide off the edge, they create SH flutter at and of the edge and release SH sound. 
