Place it at the climax of a reveal sequence, with Sudden Suspense immediately before it and Sad Violin straight after it when the scene cuts to the aftermath, or Oh My God Wow after it when another person joins the reaction.
About this sound
A Aiiiiiii is a two-second vocal clip of a single voice holding one long, open 'aiii' cry. It feels startled and over the top, like a reaction that has gone past surprise into comedy, and it lands as funny because the cry keeps going longer than a normal gasp would.