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.
Practicing And Performing Music
There is a perception that performing and practicing music are two very different activities. I think that the relationship between performing and practicing is deeply intertwined and at a certain level the distinction between the two virtually disappears.
