STEM (Science-Technology-Engineering-Mathematics) education has received great attention in recent years not only for promoting interest and learning in these areas but also for encouraging children and young people to pursue careers in them. This research explored the effects of a STEM program in developing the primary pre-service teachers' Content Knowledge (CK) and Pedagogical Content Knowledge (PCK) about sound. A qualitative and interpretative study analyzed the impact of a STEM program on the CK and PCK of 18 pre-service primary teachers that were attending a master's degree program in a Portuguese higher education institution. The data was collected from their lesson plans, field notes, a focus group interview, and participants assignments throughout the STEM activities carried out. Findings revealed several scientific misconceptions and weaknesses in the participants' PCK. Nevertheless, there was a clear positive impact on pre-service teachers' CK and PCK, specifically regarding the principles underlying STEM integration that was proposed in the conceptual framework.