Zack Snyder would have brought back Hans Zimmer to compose the score for Justice League, but the Batman v Superman: Dawn of Justice composer had already announced his retirement from superhero movies. He opted for Junkie XL, but when Joss Whedon took over as director, he brought in Danny Elfman, with whom he had collaborated in Avengers: Age of Ultron. At that time, we didn't have much information about Elfman’s approach, but a recent interview with Billboard helped clarify some things.
During the interview, Elfman mentioned that he’ll use John Williams' classic theme from Richard Donner's 1978 movie Superman in Justice League:
There are a few little fan moments. I instated a moment of the Wonder Woman theme that Hans Zimmer did for Batman vs. Superman, but I also had two minutes where I had the pleasure of saying, "Let's do John Williams' Superman."
Williams' theme was used in subsequent live-action Superman projects, including Bryan Singer's Superman Returns and the television series Smallville. But Snyder avoided using it in Man of Steel, along with other aspects of Donner's movie. Elfman will incorporate the upbeat theme into the Justice League score, but not for an equally upbeat scene. Rather, we’ll hear a darker version of the theme in one of the movie’s darker moments:
And that for me was heaven, because now I have a melody to twist, and I'm using it in an actually very dark way, in a dark moment. It's the kind of thing that some fans will notice. Some won't. It's a moment where we're really not sure whose side he's on.
Some fans might consider that last sentence a hint that Superman will return from the dead, wearing his infamous black suit. But to be honest, it's hard to tell. Warner Bros. has literally kept the superhero out of sight, and they continue to do so even now, with under seven weeks to go until Justice League rolls into theaters.