The purpose of this study is to design and develop a recorder duo educational app for elementary school students using App Inventor. The app developed in this study provides sound sources of recorder duets, and piano accompaniment in a slow tempo for practice and a fast tempo for performance. Total 11 pieces for recorder duets were chosen from music textbooks for grades 3-6 of elementary school. Using these sound sources, students can develop their "inner hearing" to imagine or listen to the sounds of counterparts while playing their own part. In addition, evaluating the developed app by four pre-service elementary school teachers in terms of content, educational utility, performance, and functionality was also executed in the study. Through this study, it is expected that teachers will be able to develop an app for learning recorder duets as needed, increasing educational efficiency and giving students opportunities for self-directed learning.