A sound output device includes: a first microphone configured to receive ambient sounds from around a user; a loudspeaker configured to output sounds toward an eardrum of the user; signal processing circuitry configured, in a case in which it is detected that an external terminal is in a playback state in which the external terminal provides a playback sound signal indicative of a playback sound, to generate a first reverse-phase signal based on a first signal derived from a sound received by the first microphone, and configured to impart predetermined frequency characteristics to the first signal in a case in which it is detected that the external terminal is in a stopped state that is not the playback state; and an adder configured to add together the playback sound signal and a signal output from the signal processing circuitry, to output a resultant signal for output by the loudspeaker.