A storage medium storing a sound processing program which causes a sound processing apparatus to function as a music playing apparatus, the sound processing apparatus to be operated in accordance with an operation input received from an input device (14, 15) operated by a player, the sound processing program instructing a computer of the sound processing apparatus to perform: an area setting step (21, S51) of setting a plurality of areas on an image displayed on a display screen, the plurality of areas being set to be adjacent to and in parallel with each other and of setting a boundary range having a predetermined width (Dmax) in a vicinity of the boundary between each two of the areas adjacent to ach other; a performance index display controlling step (21, S53) of displaying performance indexes (ST1 to ST 6), stored in a memory, each of which indicates a range of each of the plurality of areas on the display screen and a boundary between each two of the areas adjacent to each other; an operation history storing step (S21, S52) of storing a history of an operation input received from the input device in the memory; an area detecting step (21, S55, S57 to S60, S62, S76, S80, S82) of detecting one of the areas designated in accordance with the operation input and for detecting the operation input which designates an area within the boundary range or an area beyond the boundary range; a sound output information generating step (21, S72, S73, S77, S78) of generating sound output information when, in the area detecting step, the operation input, which designates the one of the areas, and crosses over the boundary range set in the vicinity of the boundary between the area and another area adjacent to the area such that the said another area is further designated, is detected based on the history of the operation input; and a sound output controlling step (21, S74, S79) of outputting a sound from a speaker (30a, 30b) in accordance with the sound output information. an area setting step (21, S51) of setting a plurality of areas on an image displayed on a display screen, the plurality of areas being set to be adjacent to and in parallel with each other and of setting a boundary range having a predetermined width (Dmax) in a vicinity of the boundary between each two of the areas adjacent to ach other; a performance index display controlling step (21, S53) of displaying performance indexes (ST1 to ST 6), stored in a memory, each of which indicates a range of each of the plurality of areas on the display screen and a boundary between each two of the areas adjacent to each other; an operation history storing step (S21, S52) of storing a history of an operation input received from the input device in the memory; an area detecting step (21, S55, S57 to S60, S62, S76, S80, S82) of detecting one of the areas designated in accordance with the operation input and for detecting the operation input which designates an area within the boundary range or an area beyond the boundary range; a sound output information generating step (21, S72, S73, S77, S78) of generating sound output information when, in the area detecting step, the operation input, which designates the one of the areas, and crosses over the boundary range set in the vicinity of the boundary between the area and another area adjacent to the area such that the said another area is further designated, is detected based on the history of the operation input; and a sound output controlling step (21, S74, S79) of outputting a sound from a speaker (30a, 30b) in accordance with the sound output information.