From what we know, Tidus and Yuna call off their relationship at some point as she sets off to defeat Sin again, who has somehow reappeared in the world of Spira. Written by scenario writer Kazushige Nojima, the voice drama takes place after the events of Final Fantasy X-2 and it along with the 2.5 novel attempt to expand the universe started by Final Fantasy X.
I'm sorry but, I already despise this idea. Really, the return of Sin? They couldn't think of something new to do?