It's okay, this isn't Unity. Audio is just HTML5 Audio tags, sprites are just HTML5 Img tags, and the 'engine' is limited to playing audio, skipping to points in the audio, drawing images, and drawing rectangles and circles.
To get them to shoot to the beat, I just have to manually set the time between shots based on the music instead of having it be random, like it is now. I don't think that process would change much even if I was using Unity. Thanks for the feedback, in a few weeks I'm going to work on making a complete version (with better sprites and maybe improved music) and I'll play around with this idea and see if it works.
The only problem I can see with this is that the enemies might all shoot at the same time, but playing around with it should work. Maybe the enemies can skip beats? I don't know. I'll find out in a few weeks and let you know.
Thanks again for the feedback.