Place it just after the piano is shown, following a short burst of Sudden Suspense to set up the reveal, then follow it with Oh My God Wow to keep the moment going. That order keeps the question in the middle of the joke, between the buildup and the bigger reaction, instead of letting it stand alone.
このサウンドについて
A short spoken reaction lasting just under four seconds, with a surprised voice asking where a piano suddenly came from. It sounds like a double-take in the middle of a conversation, curious and a little baffled rather than angry, and it works best when something appears out of nowhere. The line comes from SpongeBob SquarePants, where the odd question fits the show's sudden, silly surprises.