The phrase "Music soothes the savage beast" is a misquote of a line from the play "The Mourning Bride" by William Congreve. The actual quote is "Musick has Charms to sooth a savage Breast, To soften Rocks, or bend a knotted Oak." The phrase refers to the power of music to calm someone who is angry or upset. The phrase is often used metaphorically to refer to any situation where music has a calming effect, not just in relation to angry people.