As we have said over and over again on this site, listening to song lyrics really isn't the best way to learn English or improve your comprehension. Lyrics are frequently ungrammatical, have a strange rhythm, are written in a certain way in order to rhyme and are frequently not well-enunciated. I can assure you that as a native speaker, I have great trouble understanding rappers - admittedly, I get most of what Eminem says but not all of it. There is no such thing (in my opinion) as perfect pronunciation. Eminem has a pretty strong accent.
I would really recommend only listening to proper spoken English. BBC news broadcasts, podcasts, webcasts, TED videos, English-speaking TV shows and films (if they're not full of slang and non-standard English).
Student or Learner