I too am trying to learn German, and I find music helps, as long as the words are clear. That said, what I've mostly gained from listening to German music is a better understanding of pronunciation of words, while my vocabulary remains abysmally limited, and far as cases go, I have trouble remembering when die, der, dem or das should be used, and generally use die or der. And I'm hopeless with grammar. I'm hoping to go to Germany as an exchange student next year, and hopefully total immersion should help me achieve fluency.