Provided is a method of outputting a velocity of an object, the method including: transmitting an ultrasound signal to an object and receiving an ultrasound echo signal returned from the object; determining a velocity of the object based on the ultrasound echo signal; displaying a spectral Doppler image showing velocities of the object over time; receiving user inputs of selecting an interval within a range of the velocities of the object shown in the spectral Doppler image and adjusting an output volume of an audible sound corresponding to velocities in the selected interval; and outputting, based on the adjusted output volume of the audible sound, an audible sound representing the velocities of the object over time.