Until now, the impact of crowdsourced and interactive music projects has been limited: the public contributes a small part of the final result, and is often disconnected from the artist leading the project. We believe that a new musical ecology is needed for true creative collaboration between experts and amateurs. Toward this goal, we are creating "city symphonies," each collaboratively composed with the population of an entire city. We designed the infrastructure needed to bring together an unprecedented number of people, including a variety of web-based music composition applications, a social media framework, and real-world community-building activities. We have premiered city symphonies in Toronto, Edinburgh, Perth, and Lucerne. With support from the Knight Foundation, our first city symphony for the US, A Symphony for Detroit, premiered in fall 2015. We are also working on scaling this process by mentoring independent groups, beginning with Akron, Ohio.