Flash - How Should One Logically Organize Rhythm / Timing

I'm working on a rhythm-based game set to a specific song in ActionScript, in which a trigger spawns on the right side of the screen and moves left. The player must hit the trigger when it touches the left side of the screen, and I'm trying to figure out the most code-efficient way to spawn triggers in sync with the timing of the song. The only thing that came immediately to mind was an update function tied to the main loop that checks if the song is at a certain second each frame, but that seems unnecessary.