What is Unsilent Night?

Every year in the US people get together for what they call an Unsilent Night. It's actually the name of a composition - first tried out on an unsuspecting public back in 1992 - and here are the ingredients of an amazing 45-minute event. A crowd of people. On a count of three, they press Play on a cellphone, a boombox or other music maker. They've all been pre-assigned audio tracks - each person gets one of four possible tracks - and the combined merging of sound makes the music happen. That's the theory - so how does it work in practice? The composer is Phil Kline and he spoke to the BBC's Fergus Nicholl.