For reasons best known to themselves Marvel have very carefully not promoted the fact that Thanos appears in Guardians of the Galaxy. If you read between the lines in some of the interviews they’re basically acknowledging that he does, but we haven’t seen any sign of him and he doesn’t get referred to directly in connection with the movie.
Never the less Thanos and his chair will be present. And while we’ve seen a little glimpse of Thanos before we haven’t heard him. It’s not clear from Latino Review’s post if it’s just the voice acting that Brolin will do or if he will do some motion capture as well. I’d say he’s a good pick for the role of the Mad Titan anyway.