What you need
Blow up one balloon and tie it closed. Hold the balloon up to your ear. Use one finger to tap on the side of the balloon at your ear. Can you hear that it makes a louder sound?
Put your mouth up to the side of the balloon and talk in a normal voice. The balloon will make your voice sound louder. Try this! Blow up one of the other balloons, but make it bigger or smaller than the first balloon. Try tapping your finger against the balloon again. Does having more or less air in the balloon make the sound louder or softer?