If that building had exploded at around midnight when he set the music BEFORE he left for the theatre, the police and all medical units would have rushed there and been unavailable for the movie theatre.
THAT was his strategy.
someone could have called the plice about the loud noise, police arrive and then the explosion..He left the door cracked open, unlocked.
I think the music came on at midnight suggesting it was on a timer and went off at or around one. The woman from downstairs did call the police and was told it would be awhile because everyone was responding to the shooting at the movie theater.
Bingo!