GameHut | Sonic 3D - Unused Knuckles Music! Seems like it was placeholder from Sonic 3 @GameHut | Uploaded 6 years ago | Updated 1 hour ago
I think I've found music in the Sonic 3D Prototype 825 that hasn't been heard yet... but I might be wrong. Anyway, I believe it is an unused tune that was going to be for the Knuckles Special Stage

Music composed by Jun Senoue

Music: "New Game Plus!" By HeatleyBros
Free Music For Your Youtube Video
youtube.com/user/HeatleyBros