I don't know if anyone knows if the original version of Melodies of Life was ever released. I have heard it in two Japanese Commercials for the game. The chorus went: A voice from the past on top of yours and mine rather instead of the lyrics that were used.

YouTube - COMMERCIAL - Final Fantasy IX (2000)(Japan) #2 (http://www.youtube.com/watch?v=iQFJCU9TLrU)

Can someone help me out?

That's strange, that is definitely Emiko Shiratori singing, and the backing music is the same, but the lyrics were indeed slightly different. I admit I've never heard that version before - it was probably a special promotional version Square recorded specifically for the commercial, and hence would be almost impossible to find (it's not on the official OST album, which has some extra tracks, including an a cappella version of Melodies of Life, nor is it on Emiko Shiratori's CD single of Melodies of Life).

Would it be on the FFIX OST Plus album by any chance, which has a lot of extra 'B' sides and unreleased tracks (as I don't own this album I wouldn't know sadly)?

very interesting find. Thanks for posting this, havent heard this version before

I checked the Plus album and its not on there either. A perfect explanation is the song promo was released before the lyrics were officially completed. What we are hearing was a version of it before it was revised. It's very possible that after the lyrics were made for the promo piece, someone didn't like how it sounded (potentially Emiko Shiratori) and had them changed before the completion of the game. Promos tend to come out even before the game is finished to get people hyped up.