I believe music is one of the "universal languages" that can be understood, felt, and otherwise "enjoyed by all"; regardless of culture, society, or other kinds of differences. In addition, music has the capacity to work on people's brains in different ways than words or other communication methods.
Music, as we know, is just a whole lot of varying vibrations that fall within our hearing ranges (for the most part anyway - some experimental music uses sub and/or super hearing vibrations).
When music enters our ears, our brains process the vibrations, and we "hear" tones and blends and riffs and beats and all the other things built into it. But we also "feel" something, do we not?
I think it's that 'feeling' we sometimes experience which might be partly how some music might be considered "language of the soul", y'know?