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.

What Should You Look For In A Good Mastering Engineer?
If you’re in the music biz, then you need good mastering before you launch your records. But, what should you look for in a good mastering engineer? Discover 5 of the most important qualities to make sure you make the right choice to give your tracks the competitive edge inside!