“Sound is the barometer of the health of the planet.”It's almost 60 years since 11-year-old Martyn Stewart made his first recording near his house in Birmingham using a reel-to-reel machine borrowed from his older brother. From that day forward, he set out to capture all the natural sounds of the world, amassing nearly one hundred thousand recordings.Now, musician and sound artist Alice Boyd retraces his steps to three locations in Britain to document how these environmental soundscapes have changed, revealing vanishing ecosystems, amplified human noise and the return of endangered species.(Photograph courtesy of Tom Bright.) With archive from Martyn Stewart's library, The Listening Planet. Location recordings and original music by Alice Boyd. A Falling Tree production for BBC Radio 4