Blowing through a straw can produce a sound because?

In the simplest case, when the velocity of an airstream passing over a sharp edge exceeds about 1m/sec, then turbulence is developed in the airstream. This we hear as a broad-band (random) noise. The airstream is no longer laminar.

This is the basis of several Musical Instruments, such as the Flute.

Each time the velocity of the air is doubled, the noise goes up by a 7th power law! An increase of 24dB for each doubling of velocity. This is an important limiting factor in the design of industrial and domestic fans.

As far as the pure tone whistle that you may produce with a straw or the top of a bottle, the random noise produced by the turbulence serves to excite the natural resonance of the receptacle. Thus the notes of a Pan Pipe also.