The land's a lie, so believe
Perhaps this means that the eorzea that we’ll be playing in is some kind of false reality or offshoot of the “real” game-world. When they call it a lie they give it a negative connotation, so perhaps a later part of the story might involve overcoming some plot to hide the “real” world.
Now open your eyes while our plight is repeated, still death to our cries lost in hope we lie defeated
This part seems to say that the same individual faces death or some other catastrophe more than once. This makes me think that either some great looming disaster was delayed through some world-wide magic (thus forcing you to face the disaster again), or your character goes back and experiences some great disaster over again. With all talk of dying, I’d be inclined to think that it’s something that’s killed you (or someone you’re echoing with), and you’ll have to go back and face it a second time.
Living sins of the past for our future is taken
Now this really makes it sound like we’d be playing in some event in the past. I’d assume that these sins were committed against your character rather than by your character, though a later line suggests otherwise. In that case, I’d guess that the sins are what caused the “plight” mentioned previously. When they say “our future is taken,” I’d assume they mean we’re dead in the future. Again, I’d say it’s really a tossup as to whether your character is the one from the past, or if your character will just be reliving some story npc’s past.
Judgment binds all we hold to a memory of scorn
It seems to say that because of some misdeed, we have to be stuck in some unpleasant memory. Considering the lines that came before, the memory is probably something that our character lives in rather than some mere recollection. Perhaps your character plays a part in something bad after all.
My conclusion:
I really think that the story could go one of two ways, but for the sake of conviction, I’ll go this way: Your character has the echo, and they’re echoing with “you.” When the garlean empire invaded, they set off some disaster. In an act of desperation, some adventurers (player-controlled) activated the echo, giving themselves a mulligan on things. So your character starts over (i.e., at the point where you start playing your character), and the influence from the echo frees you from your previous fate and gives you a chance to redo things. Even so, the true reality, the one we escaped from when we got the echo, still persists in some way, and in the end we’ll have to reconcile the past world with the real world in order to go on. In the “real” world, our character was about to die, so we’ll go to the point where they were about to die and instead we’ll save them.
There are still a few holes in my guess at the story, such as: What happened to the garlean empire, and what was the sin committed by the adventurers? I figured it was the adventurers that caused the echo since the song says that something the adventurers have done causes them to have to relive things. The beginning of the song also encourages them to be free though, so freeing themselves from their fate shouldn’t be a sin.