I have that sometimes, where a song comes back, something I haven't heard in ages..
So then, I look up the lyrics, because I don't (contentiously) know or remember then.
Usually, the song turns out to be the perfect illustration for whatever is going on in my life or inside of me at the time, which means... Some part of me actually did know and remember the lyrics. I just wasn't aware of it. Same things happen with all kinds of facts too, knowledge. When I need certain information, it mostly just shows up, comes back, and the knowing seems as simple and obvious as the fact that Tuesday comes after Monday.

I'm glad you are excited about the music challenge! :)