Okay, I know that I had this kind of idea before but bear with me.

Idea Prompt 73: Anything can Happen in Music

So, we all know that Cleo (though I can never catch her streams), Wels, and Ren (I can see those videos from his streams of him singing) can all sing. The point is that after Cub rescues Wels from Season 6 (while its slowly crumbling apart). A strong surge of corruption (from you know who) blasts into Season 7 as Cub and Wels leave. 

 This causes the code of all the hermit's voices to be slightly altered. Now, you know who, can turn things into a song at any moment for his own enjoyment (he also has the help of Arthur for some more fun). 

 It's up to the trio of hermits to go back to Season 6 and stop the evil with the power of music. Woo!