mondegreen is in AHD4, which gives this: A series of words that result from the mishearing or misinterpretation of a statement or song lyric. For example, I led the pigeons to the flag for I pledge allegiance to the flag. here is malapropism from the same: Ludicrous misuse of a word, especially by confusion with one of similar sound.

so the source of both is in the hearing and the tetrapyloctomy comes mostly with words v. word.