Place it as the first reaction beat after a reveal, before you escalate the bit. Follow it with the ara ara sound for a second anime reaction, or switch straight to the nope meme sound when the character wants out instead of answers.
About this sound
This is a sub-second voice clip of Rachel Alucard saying 'nani sore,' Japanese for 'what is that?' in a prim, puzzled tone. It feels politely baffled rather than shocked, like an aristocratic character encountering something beneath explanation. The line comes from Rachel Alucard in the BlazBlue fighting games.