Whenever you speak or make a noise, your larynx (voice box) vibrates and moves the air around it in your throat and mouth. We call those air vibrations sound waves. Put your hand gently on your throat and make a noise with your voice. You should be able to feel your larynx vibrating in your throat. Sound waves travel through the air and into your ear, where they are sent as nerve signals to your brain so you can hear. To find out more about how this works, watch this BBC Operation Ouch video.
When you use your tonoscope, the vibrations of your voice travel along the straw into your tube. The sound wave then travels through the air inside the tube and hit the plastic covering the top of the tube, making it move.
The louder the noise, the bigger the vibration produced. When you make a loud noise, your grains will jump higher when they vibrate, and lower if you make a quieter sound.
The higher pitched the noise, the faster the vibration produced. If you make a high pitched noise, your grains will move faster, and a lower pitched noise will make them move slower.