This chapter examines the nature of sound and its effects on human beings, both positive and negative. It provides some sense of the vital importance of sound, hearing and listening to humanity, revealing why matters sound far more than most people think, and why it pays to spend the time to understand its impact in our world. The renowned nature recordist Bernie Krause proposes three classes of sound: geophony; biophony; and anthropophony. In human-inhabited locations, the most prevalent and significant form of biophony is usually birdsong; a sound which researchers are discovering is actually therapeutic and beneficial for human beings. The gentler geophonic and biophonic sounds of wind, water and birds are almost universally considered pleasing by human beings. Sound, particularly music, has been proven to be effective as a treatment for pain relief, asthma, dementia, stress, depression, stroke and autism, and there are academic journals dedicated to researching the effects.