Studies have suggested that musical play fosters children’s musical development and supports various educational goals (e.g., Niland, 2009; Young, 2018), leading to its inclusion as a vital component of early childhood education (ECE) curricula worldwide (Pyle & Danniels, 2017). Musical play involves children engaging in activities where they interact with music and elements such as movement, games, and storytelling, enhancing their diverse experiences in early childhood classrooms (Marsh & Young, 2006)In Hong Kong, the ECE curriculum guide formally recommends incorporating music as a core daily activity, mandating the integration of play into the curriculum, which includes musical play (Curriculum Development Council [CDC], 2017). However, early childhood teachers in Hong Kong are trained as generalists and receive limited training in music education and the integration of play-based approaches. Studies have reported low self-efficacy among Hong Kong early childhood teachers in teaching music compared to other areas in ECE (Author, 2022; Kong & Xiong, 2025).Self-efficacy beliefs significantly impact teachers’ pedagogical practices, shaping their motivation, resilience, and effectiveness (Calkins et al., 2024). Adopting Bandura’s (1997) theoretical framework of the four sources of self-efficacy, this study examines the factors that influence early childhood teachers’ beliefs and practices regarding musical play within the ecologies of early childhood music. This research is part of a larger project utilizing surveys, interviews, and observations on implementing musical play in Hong Kong. This paper focuses on semi-structured interviews with early childhood teachers (n=18) who participated in the survey. They were divided into high and low self-efficacy groups based on their scores to gain insight into factors leading to different self-efficacy beliefs. The interviews include five sections based on Bandura’s (1997) sources of self-efficacy theory: (i) enactive mastery experiences, (ii) vicarious experiences, (iii) verbal persuasion, (iv) physiological and affective states, and (v) general reflection.The findings suggest that all four sources of teachers’ self-efficacy (Bandura, 1997) are present in the context of musical play. Consistent with the theory, enactive mastery experiences (including music classroom experiences, relevant training, and playing musical instruments) are the most influential source of self-efficacy. Importantly, this study expands the literature by identifying additional self-efficacy sources in the context of musical play in ECE, including (i) curriculum resources, (ii) preparation time, and (iii) physical space. This paper concludes with suggestions for targeted teacher training and supportive environments that foster effective implementation of musical play within the ecologies of early childhood music.