Mondegreens are a sort of aural malapropism. Instead of saying the wrong word, you hear the wrong word. The word mondegreen is generally used for misheard song lyrics, although technically it can apply to any speech. They are oronyms (homophones).
The term mondegreen was originally coined by author Sylvia Wright, and has come to be quite widely used. As a child, Wright heard the lyrics of The Bonny Earl of Murray (a Scottish ballad) as:
Ye highlands and ye lowlands Oh where hae you been? Thou hae slay the Earl of Murray And Lady Mondegreen.
It eventually transpired that Lady Mondegreen existed only in the mind of Sylvia Wright, for the actual lyrics said that they "slay the Earl of Murray and laid him on the green."