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.

How Does A Music Synthesizer Work?
A music synthesizer makes sounds by using an electrical circuit as an oscillator to create and vary the frequency of sounds in order to produce different pitches.