A musical sound processing apparatus, a musical sound processing method, and a storage medium capable of generating musical sound full of interest are provided. The musical sound processing apparatus includes a first control unit configured to control a timing of sounding of a first tone in steps that come with an interval therebetween and a second control unit configured to control a timing of sounding of a second tone following or overlapping the first tone according to a first tempo, wherein the first control unit is configured to control the timing of sounding of the first tone according to the first tempo when timing information has not been acquired from outside and control the timing of sounding of the first tone according to a second tempo which is based on the timing information and different from the first tempo when the timing information has been acquired.