The songtexts are actually poems; you can find most of the lines from them in the OS but not all. For ex., The Ents is in Isengard Unleashed (17) starting at 2:18. (it stops at dambedir...) The song before it on that track is Haldir's Lament. Also, you can hear two lines from The Missing in Helm's Deep (12), starting at 3:26. "Héo dréag..." There are also more than 6 songs w/ lyrics in TTT OS. Goto www.elvish.org/gwaith/movie_soundtrack_ttt.htm to see the lyrics for every line in the soundtrack
All of the poems/songs are in whole, and blue lines represent lines in the poem that are not in the OS. Green lines can be found in the OS. Hope this helps!