Described are methods, systems, and apparatuses, including computer program products, for dynamically determining a musical part performed by a player of a rhythm-action game. In one aspect of a rhythm-action game, microphones are not tied to a particular part and therefore any player can play any of a number of parts, e.g., melody or harmony, lead or rhythm, guitar or bass, without switching instruments. This is accomplished by displaying, on a display, a plurality of target music data associated with a musical composition, receiving a music performance input data via the input device, determining which of the plurality of target music data has a degree of matching with the music performance input data, and assigning the music performance input data to the determined target music data.