A flute blows a rapid jet of air across the embouchure hole. The pressure inside the players mouth is above atmospheric (usually 1kpa: just enough to support a 10cm height difference in a water manometer). http://newt.phys.unsw.edu.au/music/

The work done to accelerate the air in this jet is the source of power input to instrument.

Sound requires an oscillating motion or air flow. In the flute, the air jet, and the resonance in the air in instrument produces an oscillating component of the flow. As the air starts to vibrate some of the energy (sound ) is radiated out the ends and through any open holes. Most of the energy is lost as a sort of friction (viscous loss) with the walls.

http://www.phys.unsw.edu.au/music/flute/

The pitch or note can be altered very slightly by breath and lip adjustment, but if changed completely the length of the air must change.

This is why the holes in the flute are used to remove the constriction of the air at that particular point.

http://www.markshep.com/flute/Acoustics.html

Since the tube walls constrict air inside the air acts like a stiff spring. This air spring receives a succession of tiny pushes and begins vibrating.

"The air stream uses energy imparted to it by these pushes to start vibrating on it's own natural rhythm. This natural rhythm is determined by the length of the air spring." http://www.markshep.com/flute/Acoustics.html

The movement of the air in the tube becomes a series of contractions and expansions. It looks like the picture of air movements below:

http://www.markshep.com/flute/Acoustics.html#Tube

Since the air spring has a constricted nature, it absorbs some of the energy that is imparted to it, so it starts to grow in strength. Before long it will over power the weak fluctuations at the mouth hole and makes its own rhythm.

"It makes the vibration build to a point at which it can vibrate the air around it, and a note is heard." http://www.markshep.com/flute/Acoustics.html#Tube