Well, I've deciphered as many as I can to the best of my limited knowledge of pop music, and have most of them done. I'm going to post it in the BH forum, for more speculation s well as possible help. I loves me some riddles.
Ah right, that does make sense, because I noticed some were like **r*** or something like that.
I wonder how you unlock them though, do you play the song? That would be a bit pointless though, so I hope it works like PGR4's Puzzle achievements, where you have to do a certain thing to unlock them.
Saw your comment in the Band Hero comments page. Pretty sure secret notes are like "Hang Man". I'm pretty sure they're band names, because I've deciphered two without trying, one being Jackson 5 and the other being Roy Orbison, both of which appear in the game.