When Edea appears in battle, I believe it's always prefaced by a sound that can be likened to a jingling of bells. That's my guess.

Here's the explanation:
Pleasant to the ears - It's a pleasant noise. Who doesn't like bells?
But deadly to your health - When you hear it for the first and second times, it's bad for you health on account of the fact that you're fighting Edea.
Only one window exists - However, Edea does join your party for one segment of the game.
Where it means salvation - So it's no longer a sound synonymous with fighting an evil sorceress.