A mobile electronic device and a sound playback method thereof are provided. The mobile electronic device includes a sensor, a speaker, and a controller coupled to the sensor and the speaker. The sensor detects whether the speaker is blocked or not. When the speaker is blocked, the controller multiplies a sound signal by a transfer function and then outputs the multiplied sound signal. The speaker plays the sound signal outputted by the controller. The transfer function changes the direction in which the speaker plays the sound signal.