The sound of the world around us provides us with key indicators to the health of our planet. How those sounds change over time and in space can show how the well-being of earth is changing, both naturally and through man's impact. Soundscape ecology is the study of nature's sounds - from the lapping of the ocean's waves and the rustle of leaves, to the rutting roars of red deer and the whistling of whales and dolphins. But the sound of our world is changing, advances in recording technology mean that we can now very easily listen to that change. But one of the key things for science though is being able to analyse that soundscape and relate it how the natural world is changing. In this episode of Brainwaves, Pennie Latin talks to those who have spent years recording the evolving soundscape of our planet, explores how the sound is being analysed and discovers how sound is being used to measure re-wilding in parts of the Highlands.