Really solid game that reminds me of hub-world collectathons like Mario 64 and Banjo-Kazooie. The movement feels smooth, the style is amusing, and it looks like there's going to be a lot of variety. I enjoyed discovering the singing mechanics; there's a lot more to it than it first seems. I'm eager to see more of this game in the future.
There are a few things that I think could use a little polishing. The button inputs seem a little inconsistent: the menu cursor and singing ability both respond when releasing the button instead of pressing it, and holding select prevents jumping from working. The enemy hitboxes feel bigger to me than collision with backgrounds, and that made me take a lot of hits right on the very edge that I didn't expect. The options screen is a little awkward with the full-screen flicker when changing options and the cursor resetting to the first entry. Finally, the wavy effect when entering a book has a random single-scanline glitch related to $2005 writes landing around dot 257 (you can test this in Mesen with Settings -> Emulation -> NES -> "Enable PPU $2000/$2005/$2006 first-write scroll glitch emulation"); writing on that dot causes X scroll to change to $20 for one scanline.