It was techno. I want to know why the cops never came to his door or how the music was stopped if they didn’t. The witness who says someone took a call in the front row of the theater, exited and possibly allowed James into the theater is another interesting angle.
It was a repeating loop. The sound system was reportedly set to a timer. It came on after Holmes had already left, apparently timed to the beginning of the movie and his attack, and it shut off via timer at 1:00AM, before police arrived at the apartment.
A downstairs neighbor, the woman who tried the door and found it open but fortunately did not go in and trigger the explosives, provided the only description that went beyond merely “techno.” She described it as having a lot of very deep bass and reverb. She is a 20 year old student, surname Fonzi.