There's a video posted below if you prefer video over text!!!
Since this will be your first cassette tape, I'll go into a bit of detail. There's a cassette tape in every main level, usually in an out-of-place room. These rooms feature alternating pink and blue platforms that are usable only when they're highlighted and unusable when they're dark. They change in a set pattern for each room. Only pink or only blue platforms will be highlighted at once, not both.
From Section 3 of Chapter 1, jump up in the first room and grab onto the moving platform to get it out of the way, then quickly dash over it and climb the wall to go up into the next room. Stand on the moving platform here and jump at its peak, then dash upwards to reach an upper platform and the next room through the ceiling. Dash up and grab onto the moving platform, then drop down and head into the previously hidden secret room that it was blocking, which is the cassette room.
Welcome! Observe the pattern and soak it in. You'll need to cross these alternating blocks as they are usable. Crossing over the two blue blocks is a little harder than the others, but if you go full speed, you shouldn't have any issues. When you get to the moving platform, I suggest holding onto the wall overhead until the pink block goes dark to jump onto the platform and fling toward the blue block. You can hold onto the blue block if it's still there, jumping at the last second to dash forward and land on the ledge with the cassette. Grab it and be bubbled back to the start of the room.
If you fail along the way, you'll start back at the beginning of the room and the cycle will reset from a different point than where you left off. Make sure you observe again before you start over.
Timestamp: 16:05