I'm surprised, honestly, that you say that you can't understand "some" songs. I mean, I still struggle with spoken German, and I've been studying it intensely for 5 years... Hell, I can't understand some English songs, even if I've seen the lyrics... I don't know if you've come across any German Pearl Jams, but I think that's the double-edged nature of learning with songs - sometimes the singer won't even try to enunciate.