Use the speaker in your smartphone buzz a certain tone, no need to buy a speaker and hook it up to Arduino, make this as the output function triggered by Arduino when a certain action happens.
Learn more from the examples page.
This shield is one of the I/O shields.
I/O shields provides simple function of using your smartphone as input or output to Arduino.
Learn more from the I/O shields tutorial.
Buzzer library allows Arduino board deal with your smartphone’s speaker and use it as a buzzer shield.
Adding #define CUSTOM_SETTINGS along with #define INCLUDE_BUZZER_SHIELD is a key that will let you unlock the Buzzer shield only amongst all shields and prevent 1Sheeld library from eating your memory specially for UNO board.