Sound output device and control method for sound output device

A sound output device comprising:

• 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 obtained by imparting the predetermined frequency characteristics to the first signal, to output a resultant signal for output by the loudspeaker.