Click the green flag, enable your microphone, and try clapping and/or talking loudly to see how your volume is used an an input for this program. See my project "Draw with Sound" for another alternative for using "loudness" as an input.
You'll need to enable your microphone for Scratch, but don't worry, nothing is being recorded. The loudness or volume of the noise coming from your microphone will be used as the input.
