Teacher Efficacy (TE) refers to teachers' confidence in their ability to promote student's learning. Research shows that generalist teachers' TE for music education is generally low, with many teachers associating music education with innate musical talents -- a problem exacerbated by music education programmes that focus on singing or playing instruments. Musical play is a form of music education in which young children more or less freely explore and create with sound with limited guidance from a teacher, requiring fewer musical skills. This paper reports on an in-situ professional development program (PD) that focused on musical play as a means of enhancing early years teachers' TE for musical play and for music education in general. For this study, a mixed method exploratory multiple-case study was used. Three early years teams (N = 14) took part in a PD that focused on knowledge and skills related to facilitating musical play. Data were collected through surveys and interviews. Findings demonstrate that taking part in the PD enhanced teachers' TE for musical play and for 50% of the respondents for music education in general. The PD's design features longer duration, collective participation, active learning and qualitative curriculum materials had contributed to teachers' learning.