I've just started really listening to Japanese music and I must say I find it very difficult to understand the lyrics. I can't catch anything until I look at the written lyrics and then as I follow along I can catch it all.
I guess it comes down to the fact that the words are being sung, so many words are not "spaced" normally, and the singer's voice is rising and falling constantly. Plus, the fact that all the music is in the background as well.
Movies are pretty hard too. Actors have the lines prepped and specially prepared so it is sometimes hard to follow. I think it's the same thing as with music because even though I am a native speaker of English, if I watch a movie from England, I sometimes have trouble catching what's being said because of different accents.
Oh well...the music is still really good even if I can't really understand it. I'm sure it gets better with practice.