Place this clip as the final reaction beat after the cute reveal lands, not under dialogue. For a bigger build, lead in with an oh my god wow reaction first, then let this sigh close the moment before cutting to the next scene.
About this sound
This is a short three-second clip of a drawn-out 'awwww' that turns into a soft, enamored sigh, like someone swooning out loud. It feels sweet, tender and a little over-the-top, the sound of being completely charmed and melting over something cute rather than just liking it.