It is important to first understand that what we are calling 'sound' is much more than the auditory sound we can hear with our ears. We as humans can only hear a specific range of frequencies or vibration, but sound is happening at much much vaster and subtler levels than we perceive with our ears. But we can still perceive this 'sound' in other ways as emotion, and even thought and consciousness are registered as vibration by different parts of our beings. When we deepen our connection to self, we become more able to perceive these vibrations. Through our intention and practice, we become like finely tuned radios, tuning in to subtle levels of vibrational perception, understanding much vaster amounts of information than is normally thought possible.
The mystics, shamans, yogis, and healers have always understood that all of creation is vibrational in nature, and vibration creates sound. Modern science now understands this as well and recognize that on unseen levels, everything is in motion, in specific patterns, frequencies and so on. When two vibrating objects come in proximity, they are attracted or repelled depending on the vibration they are emitting. So when you see it as sound, there is harmony, or not, depending on their vibrational relationship. One example is how sometimes you enjoy being around certain people and not others. Sometimes we understand why and sometimes we don't, but we can feel this vibrational relationship. It doesn't mean theres anything wrong with either person, it just means you are on a different wavelength that is not in harmony, just like certain notes sound good together and others do not.