I also think that it can be found on Track 3 of the RotK OST "Minas Tirith", starting at 2:02. See Magpie's site for the track breakdown by when each part was in the movie
here.
The poem is called "the White Rider" and goes thus in Sindarin:
Mennen nored dîn, Gwanwen i 'ûr bân, Sílant calad Dûn, Tollen Rochon 'Lân, though they're in different order in the actual song.
Not mentioning what I think of certain aspects of the translation / pronunciation....[/b]