A mondegreen is a mishearing or misinterpretation of a phrase as a result of near-homophony, 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 clearly hear a lyric, substitutes words that sound similar, and make some kind of sense.
American writer Sylvia Wright coined the term in her essay "The Death of Lady Mondegreen",
published in Harper's Magazine in November 1954.
The term was inspired by "...and Lady Mondegreen", a misinterpretation of the line
"...and laid him on the green" from the Scottish ballad "The Bonnie Earl O' Moray".
An unintentionally incorrect use of similar-sounding words or phrases in speaking is a malapropism.
If there is a connection in meaning, it can be called an eggcorn.
If a person stubbornly sticks to a mispronunciation after being corrected,
that person has committed a mumpsimus.