Mixtapes have physics behind them but you have to look at in a different perspective. Mixtapes have wave interference which is two or more waves in the same medium at the same time and same location. Mixtapes use sound waves and a lot of times artists who make mixtapes choose two waves that start at the same time to start their song. A new sound wave might interfere but they both continue as if they have never met and that represent constructive interference. If there is a destructive interference there will be a smaller amplitude which means it will be a softer sound for the mixtape.