B-Side Cassette Tapes (or simply Cassettes for short) are collectible items, one of which can be found in each of the story chapters. Obtaining a chapter's Cassette is required in order to unlock the chapter's B-side level, which uses the mechanics of its respective chapter but is significantly harder. Cassettes are always obtainable in their own room after a gauntlet of Rhythm Blocks. The following page details how to obtain these Cassettes.
Chapter 1: Forsaken City
This chapter's Cassette room can be found in this chapter's third sub-chapter, "Chasm". If the player goes behind the Traffic Light Block in the sub-chapter's first room, then goes two rooms up, and then goes behind the next Conveyor-belt Platform, the Cassette room will become accessible.
Chapter 2: Old Site
This Cassette can be found in this chapter's first sub-chapter, "Start", but only after the mirror encounter with Badeline. The player must go down as soon as they enter the large room (the first one where the player has any agency over movement), and must then dash down through the Dream Block in the floor of said room, exposing the Cassette room.
Chapter 3: Celestial Resort
This Cassette can be found in this chapter's third sub-chapter, "Elevator Shaft", just to the left of the chapter's singular Key (indicated by the opening through a broken wall with light shining out of it).
Chapter 4: Golden Ridge
This chapter's Cassette can be found in this chapter's first sub-chapter, "Start". Just after finishing the room that requires the player to collect two Shields in order to move a Shield-Activated Platform, the player must bounce on the second White Cloud from the left, then dash up and left in order to access the top-left corner of the room. The player must then go as far left as possible, passing through the following room (which should contain a number of Springs and strawberry fragments in order to find this Cassette's room.
Chapter 5: Mirror Temple
This Cassette can be found in this chapter's second sub-chapter, "Depths". After passing the third Temple Gate—the one just after a challenge involving a single Swap Block—the player should head down and to the left in order to find a Red Orb. Taking this orb to the right will lead to a platforming segment; completing this segment and pressing the Dash Switch there will open another Temple Gate to the bottom right of this room. Clearing this second room by depressing its Dash Switch will open yet another Temple Gate at the bottom right of this following room, leading to a third, more illuminated room. At the bottom right of this third room, the player can simply walk through a section of cracked wall (with or without completing any associated challenges) to access the Cassette room.
Chapter 6: Reflection
This Cassette can be found in the first room of this chapter's third sub-chapter, "Hollows". To access the Cassette room, the player must use the nearby Kevin Block to break the wall below it and send the Block to the bottom of the lake. The player must then quickly attack the Kevin Block again to make it break the wall to its left, exposing the entryway into the Cassette room.
Chapter 7: The Summit
This chapter's Cassette can be found in its fourth sub-chapter, "1500M". Upon entering this sub-chapter's fifth room, an opening can be seen leading out the top of the room. Entering this opening will lead to the cassette room.
Chapter 8: Core
This cassette lies at the end of the chapter and is required to complete it.