I believe the answer is music, or more specifically the Highwind Theme.

Quote Originally Posted by musenji
Find me by land, or much later by sky
When doom is at hand, the first option will die.
After Meteor appears, the Highwind theme can only be heard while actually flying it. It could, however, be heard previously.

Twice I'm there when you've escaped via rope;
Once while unconscious and regaining hope.
It applies to these instances, that is the two rope escapes and the Lifestream incident.

It should be noted, I've more than one form--
Sometimes I'm chilling, but usually I'm warm.
The tone or form of the music can change. Usually, it's "warm."

If you are dextrous, then carry me well
To coax divine pow'r from an old corpse's shell.
Playing the piano (as previously revealed, the "old corpse's shell) skillfully (or at least accurately) will "coax" forth the divine power, Final Heaven.