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.
Music Piracy and the New Musician
This article discusses music piracy and the hazards that it represent to all musicians. It discuses the nature of this piracy and its estimated costs and outlines the importance of protecting ones intellectual property.
