Aah, a music game. I love music games.
I once had a similar idea, using several tracks and let the player "construct" the music, but I had them draw shapes with it (which had to roughly match the tone of the music, as far as speed goes (straight lines drawn quickly, etc). That turned out to be too complex, and not much fun. Yours is simpler, but I fear that its too simple - so far not much more than clicking things. Bah!
So here's what I'd try to do with it: Make it not only a music, but also a rhythmgame.
For each track (apart from the vocal track), it should be relatively easy to find the "highlights", i.e. the moments when the instrument is played, say when the drum is hit. As long as the player hits the yellow spheres randomly, the track can be heard, but it only ever reaches, say, 30%. At this point, the player has to try to hit the spheres in rhythm (add classes ("Perfectly hit!", "Good hit!", "Barely!", etc) whose effects have different strengths, maybe also give special bonuses if I manage to hit a complete sequence perfectly (i.e. every bass note in one part)). To make this easier, spawn less spheres.
The vocal track seems to be the hardest to turn into gameplay - maybe you could work with the pitch of the voice (the player has to "waggle" the mouse cursor in a speed relating to the pitch the player has to draw circles around it with a speed related to the pitch?)? Or maybe, the action one has to do relates to the lyrics?, or maybe something entirely different!?
A round would then, obviously, be one song.
If you go with that (please do!), then remove the black blops, or make them less frequent.
Also, add a combosystem with clear, huge visual indicators of my score and my combochain.
And don't only make the music react to the actions - also change the background accordingly. You guys are the master of abstract nice designs, so I'm sure you can figure something much better out than I ever could!
