Quest for the 7 Keys

Ok, this has been on my mind for a while. I have downloaded all of the secret songs and absolutely can not figure out what the "keys" in each song are! I know that the contest is over and happened like over a two years ago, but it still has me curious. So, what were the keys in the secret songs, how were they used to unlock the doors, and what were behind the doors to create the ultimate key?