The purpose of this study is to introduce the SoI-EY framework and investigate infants’ spontaneous musical behavior based on that framework. The SoI-EY framework is an instrument for investigating children’s musical behavior and engagement. It was originally developed in England for children with learning difficulties, but a growing body of research using the SoI framework has been conducted throughout the world. SoI-EY was developed in order to explore the potential relevance for infants of so-called neurotypical musical development. We investigated three infants’ musical behavior on the basis of that framework, gathering data by recording video over 13 weeks, then analyzed the total 1693 minutes of video. The frequency, average, and percentage of musical behavior were conducted based on three domains of the SoI-EY framework. We observed that Boy A ranged from Level 2 to Level 4 in the three domains, showing Level 2 overall, while boys B and C fit Level 2 in the reactive domain, and Levels 2 to 4 in the interactive and proactive domains. The results indicated that children of similar ages have different musical development levels, and that a child’s environment, developmental level, and playmates can all influence the level of musical development.