The game uses the voices of the actors from the films. You're hearing Cate Blanchett as Galadriel, Hugo Weaving as Elrond, Andy Serkis as Gollum and so on. The game's version of J. R. R. Tolkien's epic is mostly faithful to the movie's scenes. On occasion, however, they deviate and have some silly fun.
In this video we compare the Lego and movie versions of the opening of Fellowship of the Ring, Gollum's first appearance and that bit with the horses made out of water.
Hats off to the game's developers at Traveller's Tales. We'll have more on how the game plays in the near future.