Before I learned much about coding (and I remain far, far from an expert), I hired some folks to help me make a music sync app. Needless to say, it was a learning experience par excellence.
I could write paragraphs here explicating the details of what exactly went wrong but the general idea can be summed up thus: Creating apps is fun; creating businesses that work is hard.
TeamSync aimed to create a synchronized music experience (like turntable.fm). A user would have been able to check out what her friends were listening to in the moment and tune in. As the app works now, a lead user can set up playlists that others follow. TeamSync does not stream music, but attempts to synchronize existing media. This is actually quite a challenge and the user experience never quite got there, but was close by the time I set this aside for other work.
What follows here is how far I got. Feel free to pick up the pieces and improve upon the app if you like, or add parts to your own project.