I've got a decent amount of playtime under my belt in Theatrhythm: Final Fantasy so far. I have over 12,000 Rhythmia accumulated, I've cleared 10 out of the available 13 Series courses, I've cleared 22 different Dark Notes, I've SS'ed several songs in Challenge mode and cleared a number of Ultimate scores.
Despite all of that, however, I've only been able to uncover one additional song, in the hidden "Encore" category. According to the main menu, this brings my total Challenge Mode song count up to 40, when the Museum mode shows that there are 77 music tracks total. I've played a number of new songs (as well as otherwise-unplayable tracks such as OP/ED themes) as a part of Dark Notes in the Chaos Shrine, but can't seem to access them outside of that mode at all.
Are there more songs I can unlock for Challenge Mode, and if so, how? Additionally, is there a way to permanently unlock Chaos Shrine songs for regular play?