Apparently the hoof's drummer Greg Saunier, producer / multi-instrumentalist, Jon Brion and Wilco avante-guitarist Nels Cline are all "playing together on the film's soundtrack," which presumably means they recorded and wrote the score together. Update: Some details just arrived. "Step Brothers" Director Adam Mckay and Jon Brion had Cline and Saunier come into a studio a few weeks ago to record some music for the film. Some of it was improvised and some of it was already written by Brion. It's unsure whether this material will make the cut yet, but it sounds like Brion is the principal composer on the film.
More.... Source: The Playlist