With a little elbow grease, skeleton stages for Clown Man and Tengu Man have been forced out of hiding, revealing more unseen (and unheard) goodness. Both stages, though incomplete, tout drastically different layouts and feature a single piece of unused music:
You can download the beta (and the unused music) within the video's description.
On a related note: I've been hearing a lot of requests for the release of beta v2. I'm currently working on that, but I thought I'd give you all fair warning: it's not as significant as v1.
v2 is near final. The build consists of five and a half stages: introductory, Clown Man, Grenade Man, finalized Frost Man, and an incomplete Sword Man stage. Each stage is nearly identical to the final counterparts, but all lack certain graphical background details. All stages lack boss battles and all end with a dead end. The only way to return to the stage select is to reset the game.
So, yes, beta v2 doesn't contain anything hugely awesome like the unused swamp introductory stage. Still, it's worthy of a release just for the sake of completion. I'll be keeping you posted on that front.