Although the character in the teaser trailer is not the character we'll be playing in the final game, our character in SOMA
will be speaking and offering thoughts on his situation and environment (as well as talking to other characters he meets in the game). According to the following preview article, the character's speech is still undergoing some tweaks:
http://www.eurogamer.net/articles/2014-0...-game-soma
That particular previewer doesn't seem to be a fan of Simon's voice, but his preview is written roughly a year before the game is expected to be finished, so there's plenty of time for FG to receive feedback and find a nice balance for Simon's incidental chatter.
While I agree that a talking protagonist has the potential to ruin the immersion, it's not like Daniel and Oswald didn't speak a fair amount in the Amnesia games already. I think the main difference there was that they were talking during "events", rather than commenting on random things which don't move the story along. If Daniel had been saying "hmm, more bookshelves..." as you walked through the rooms, that would have been annoying and immersion-breaking. I hope Simon doesn't do that.