On a level where it matters, I muted the sound and the game concluded I cheated. Clearing my cache and replaying it happened again, so I am guessing you aren't supposed to do that, but it's weird because the sound volume can't matter on any other level (that is, there seems to be custom behaviour that makes it work on this level) I'm not sure how the cheat detection works, obviously, but it is pretty annoying. I would guess it was intended if the overlay went away after fulfilling the instructions, but it doesn't which makes the game pretty unplayable (or, at least, unfun).
In the time it took to type this and double check my assertions the overlay has now disappeared, so maybe it's not cheat detection? It also triggered (and was fixed by clearing cache) when I guessed the wrong answer on a puzzle regarding pixel colors (I thought maybe you needed to change the target number via editing the page), which I assumed meant web page edits weren't in the intended solution space, though, so I'm pretty sure that's not supposed to happen...
Edit: Probably should have read the instructions. I was trying to avoid spoilers, but it appears inspect element (and, I assume, page editing generally) is cheating. That still doesn't explain why it would trigger just from scrolling the sound down, though.