A sound-processing apparatus is provided that is capable of creating a sound effect without having to worry about changes in parameters such as the beat of the input sound source. The sound-processing apparatus comprises: a delay unit DB that generates at least one or more new synchronized sound signal that is synchronized with a sound signal Si to be modulated; parameter-detection unit 6 that detects a parameter that indicates an attribute of the generated synchronized sound signal; a signal-conversion unit 8 that generates a converted signal Sdm based on the detected parameter; and a modulation unit that modulates the sound signal based on the generated converted signal Sdm.