An audio output device includes a voltage detection unit, a processing unit, a voltage detection unit, an alarm unit and an audio output unit. The voltage detection unit is utilized for detecting a voltage level of an audio signal to generate a detection result. The processing unit is utilized for determining whether the voltage level of the audio signal is greater than a threshold value according to the detection result and accordingly generating a control signal. The volume adjustment unit is utilized for adjusting the audio signal to generate an adjusted audio signal according to the control signal. The alarm unit is utilized for outputting a first alarm signal to implement an alarm function according to the control signal. Then audio output unit is utilized for playing the adjusted audio signal according to the control signal.