What’s Going On?
The beveled “lips” you cut into the squashed end of the straw act as a reed for your instrument. When you blow into the reed and get it vibrating, you send pulses of compressed air down the straw, causing the air in the tube to start vibrating, too (click to enlarge diagram below). Affected by the length of the tube, this vibrating air in turn affects the reed’s vibrations. When the reed vibrates at just the right frequency, the air in the straw vibrates powerfully, and you hear a loud, buzzing note, sort of like an oboe.