The Avengers: Age of Ultron that I didn't mention in my original review was its score. That's a story unto itself -- though one where I'm missing some of the details.
Months before the film's release, it was announced that Brian Tyler would be composing its music. For score enthusiasts, this was both surprising and not. The first Avengers had a solid score by Alan Silvestri, so it was odd that he wasn't returning for the next installment. On the other hand, the Marvel Cinematic Universe has changed composers quite often. And putting Brian Tyler (who had previously done Iron Man 3 and Thor: The Dark World) on Age of Ultron already made for more musical continuity than Marvel typically had in the past.
Then the first poster for the movie came out, and eagle-eyed fans immediately noticed a credit of "Additional Music by Danny Elfman." In the finished film, this went one step farther; Brain Tyler and Danny Elfman are both given standard "music by" credit. What appears to have happened is that someone on the film (likely a Marvel producer) disliked Tyler's submitted score -- but not so much to toss it out entirely. Why Marvel was willing to pay Danny Elfman's presumably high price tag when they hadn't paid Alan Silvestri's in the first place is a bit of a mystery. Perhaps Silvestri turned them down for reasons other than money? (Joss Whedon has certainly been implying that Marvel movies are an increasingly difficult place for creative types to work.)
Whatever really went on behind the scenes of Age of Ultron, the result is an interesting score that represents the work of three composers. Brian Tyler and Danny Elfman each wound up on roughly half the movie, and each of them also repurposed the "Avengers theme" written by Alan Silvestri for the first film. Yet where you might expect that to yield a schizophrenic end product, the three sounds all blend together pretty well. You'd have to be a real film music nut (more even than me) to guess who wrote which cues, and you'd probably still get it wrong some of the time.
Generally speaking, Brian Tyler gets more of the action music. His "Avengers: Age of Ultron Title" opens the soundtrack album with a fast, frenetic piece that builds a sense of dread. He toys with wild sprays of jungle percussion in "Hulkbuster" and "The Vault," pulsing bass in "The Mission" and "Soeul Searching," and majestic choir in "Rise Together." Still, he's not high octane all the time. He plays with a slow, almost James Bond sort of vibe in "Breaking and Entering," and amorphous, free time music in "Birth of Ultron" and "Vision."
Danny Elfman (again, generally speaking) gets more of the music with emotional heft. His track "Ultron / Twins" brings an almost gothic quality to the former character, and a distinctly Russian tone to the latter two. He uses acoustic guitar in an unabashedly sentimental way for "Farmhouse" and "The Farm." And it's his reworked version of the Silvestri Avengers melody that soars most often throughout the album, on "Heroes," Avengers Unite," and "It Begins" (the last of which is vaguely reminiscent of Elfman's Batman work, if you strain to listen for it).
The Age of Ultron soundtrack serves up a lot of pulse-pounding, triumphant tracks, but with just enough subtly deeper work to make it stand out a bit from other action fare. I give the album a B+.