Woodwind instruments produce sound by the vibration of air flow. Generally, there are two vibration modes. If it is the simplest woodwind instrument, when blowing inside, the air that enters and passes through the "blowhole" will hit some parts of the pipe, and through the length of the pipe, the vibration of the air will be transmitted to make a sound. If it's other woodwind instruments, they all have reed. The air entering the "blowing hole" will make the reed vibrate, and cause the air in the tube under the reed to vibrate, so the sound will be heard.