Quote Originally Posted by Old Manus
And since when could Yuna SING?
Notice that when she sings its always in the songstress dressphere. Its because of the songstress dressphere, just like Rikku, Paine, and LeBlanc are all able to sing with it. Its due to the inhabiting spirit of the songstress dressphere, the spirit of (SPOILER)Lenne, who was a glourious and famous singer 1000 years ago in Zanarkand, that they have said ability.

Of course, you'd actually have to pay attention to the story and plot of the game to know this, which so few people tend to do for some reason. Probably has something to do with that pause, select + square button thing I assume.