This research aims at a music piece for audience participation using mobile phones as musical instruments in a music concert setting. Inspired by the ubiquity of smart phones, I attempted to accomplish audience engagement in a music performance by crafting an accessible musical instrument with which audience can be a part of the performance. The research begins by reviewing the related works in two areas, mobile music and audience participation at music performances, builds a charted map of the areas and its intersection to seek an innovation, and defines requisites for a successful audience participation where audience can participate in music making as musicians with their mobile phones. To make accessible audience participation, the concept of a networked multi-user instrument is applied for the system. With the lessons learnt, I developed echobo, a mobile musical instrument application for iOS devices (iPhone, iPad and iPod Touch). With this system, audience can download the app at the concert, play the instrument instantly, interact with other audience members, and contribute to the music by sound generated from their mobile phones. A music piece for echobo and a clarinet was presented in a series of performances and the application was found to work reliably and accomplish audience engagement. The post-survey results indicate that the system was accessible, and helped the audience to connect to the music and other musicians.