If you can understand him (Spanish), here is an interview about his (Gustavo Santaolalla) involvement in The Last of Us.
-He's very excited about it, it's something he has not done before (videogames) and it requires a different approach: he mentions it's a year of work.
-He's under NDA (so not giving specifics). Mentions it is not a typical game, but an atypical one.
-From Sony and the makers of Uncharted obviously.
-It has what you would expect from a vg (combat scenarios) but carried by a very engrossing and emotive story and that is what makes it different from other videogames.
-He loved the project and the possibility to work on it so he took the job.