The general term for a misheard lyric is a "mondegreen," a word originally proposed by Sylvia Wright in a 1954 essay in Harper's Magazine. It's one of the few deliberate coinages to make it into dictionaries. Wright derived the name from something she mis-heard as a child, specifically part of a ballad called "The Bonny Earl O'Moray." Instead of "They hae slain the Earl O' Moray and laid him on the the green," she thought the words were "They hae slain the Earl O' Moray and Lady Mondegreen."
One of my personal favorite mondegreens was pointed out by Dave Barry, who noted that the opening words of "Help Me Rhonda" sound a lot like, "Ever since she left there've been owls puking in my bed."
Some reserve the meaning of "mondegreen" for mis-heard lyrics in the same language and prefer to use the Japanese word soramimi when foreign words are mis-heard as being in one's own language (as with "O Fortuna" above). On YouTube there's a tradition of subtitling foreign language music videos, especially from India, with sound-alike English lyrics, sometimes with hilarious results. This is commonly called "Buffalaxing" after someone who posted several early popular examples under the name Buffalax. (It helps that music videos from India, often segments from feature films, tend to be very entertaining in their own right.) Search YouTube for "Buffalax" or "Buffalaxed" and you'll see plenty of examples. (Be warned that the subtitles are sometimes a tad naughty.)by