These songs are unlocked for the Chaos section of the Challenge series. Each track has its own special difficulty score and are unlocked by gathering Rhythmia.
Go to Museum -> Collection -> Password and enter the desired password to unlock the CollectaCard. Each password can be used only once and they are case sensitive. If you already have the card unlocked, entering the password will add one level to the card's level.
By earning more and more Rhythmia, you can earn colored Crystal Shards to help you unlock more characters.