Clear the game's final stage to unlock Sound Test mode on the Main Menu. Sound Test allows you to listen to all of the songs in the game, as well as a few that didn't make it into the game.
After you clear the game's final stage, you'll open up one more location on your map: Sal's Hut. Sal's Hut allows you to see 3D models of all of the Grocks in the game, as well as the Rangers. You can access it by earning all 70 Stars from the stages
Sal's Hut - Beat the game and earn all 70 Stars