This is not part of the main game. What you need to do on the main menu is go to the extras or music room (can't quite remember the name).
Once there, you'll be in the first cortex room. You should be able to press the button and transition to the next space each time (not clear if you need to play the instruments or listen to the music for a set amount of time).
Edit: May be with a spinning cube.
Leave a comment if I missed something or if you have more clarity. Glad to update the solution.