A sound-measuring device for measuring average amplitude of sound waves of a sound-emitting device is provided. The sound-measuring device includes an integration circuit, a laser diode, a driver configured to drive the laser diode to emit a laser beam, a storage unit, a light sensing unit, a first optical fiber and a second optical fiber aligned with the first optical fiber. The first optical fiber is placed on the sound insulating film. The sound waves of the sound-emitting device causes the insulating film to vibrate thereby the vibration of the insulating film causing misalignment between the first optical fiber and the second optical fiber. The integration circuit calculates an insertion loss value between the portion of the laser beam and the laser beam from the laser diode, and determines average amplitudes of the sound waves according to the table and the calculated insertion loss value.