If you sing a cover song and change something about the track (whether it be lyrics, timing, key), your cover has now become a derivative work. Think of it as deriving from another piece of music and, therefore, become something new. You will ALWAYS need permission to do these.
Making Your Own Music: Songwriting Basics
Songwriting is an excellent way to express your thoughts and feelings and to communicate them to your friends and family, even if you have no aspirations of musical fame or fortune.
