Known as mondegreens:
A mondegreen is a mishearing or misinterpretation of a phrase in a way that gives it a new meaning. Mondegreens are most often created by a person listening to a poem or a song; the listener, being unable to hear a lyric clearly, substitutes words that sound similar and make some kind of sense. The American writer Sylvia Wright coined the term in 1954, recalling a childhood memory of her mother reading the Scottish ballad "The Bonny Earl of Murray" (from Thomas Percy's 1765 book Reliques of Ancient English Poetry), and mishearing the words "layd him on the green" as "Lady Mondegreen"