Clicking on Tia makes her move and sing, and the script to do this can just live in her sprite with a "when clicked" cap.
Clicking on a button, however, can't make another sprite move or change costumes without sending a message. The buttons in this project accomplish this task by broadcasting messages. We format messages like this:
It's just a convention in Scratch, but it is syntax in other languages that you might as well get used to.
Duplicate a button. Edit the scripts so that the new button does something interesting with one of the instruments.