Canonical:
His own theme, of course. Sorry, could not find the actual OST's cover, so the fanart will have to do.
Endless Departure, the song that plays over that oh-so-defining centuries-long search for the tower.


Pertaining to:

Canon:
Skillet - Hero.
Red - Hymn For The Missing. Seriously, this is going to happen to him at least twice, all told.

Meridian:
Poets of the Fall - Diamonds For Tears
More belongs here but I can't think of them right now.

Animus:
Hypnogaja - Blue
Coldplay - Lost?
Linkin Park - The Catalyst
VAST - Lost by suggestion.
I know these seem a bit contradictory with one another, it's complicated.

I probably missed a lot. Feel free to add suggestions?