I bet you could play some of the levels blind.
This is true of almost all of the levels. Aside from the initial pass through to learn the controls, everything can be done by simply listening. As a matter of fact in some levels I HAVE TO close my eyes and play or else I can't do it. It's a pretty bizarre experience.
Overall, I'm enjoying the game thus far. It's very addicting and it's really quirky and unique. For someone like me I'd gladly drop $30 on it because these offbeat titles are right up my alley. Others may not find it quite as endearing, but the rhythm gameplay is definitely unique even amongst a sea of similar titles.
For those who have played the demo, keep in mind what was showcased there was only 1 of many mini games. You'll definitely be surprised by some of the cool ideas they came up with for the mini games.