We've all been blown away by singers before. Some people can sing with technical skill, but also a range of emotional resonance that touches us on a deep level. We think, wow, it's incredible that someone can make that sound. There are many types of song, from classic opera to the death metal growls. Whatever your taste, all kinds require training and some level of technical expertise. Then there's polyphonic singing. That's when one person creates two different tones with one breath--two notes coming out of one person's mouth. "Polyphonic" means "multiple sounds," and in instruments, it's like playing tow...