Right now I'm just working with frequency waves, so it's a challenge to make big variation on gameplay. It does affects both spawn rate of enemies and their speed, so calmer songs should be a little easier than faster/louder ones...but that's it. I'm thinking about adding some enemies variations based on the frequency band, but I'm still kind of limited by frequency. If I were workign with midi files than yeah I could do WAY more stuff, using music notes and more, but then people wouldn't be able to just upload their .mp3s to play, so that's my limitation right now. Also maybe I will try to synch the shooting with the beats of the music, but I'm not sure how to do that yet lol