I think it could be
a spoiler
Those who already know can't hear.
if you know about something, when you hear it you aren't hearing a spoiler
Those who can hear already know.
If you can hear a spoiler, you know about it
What do you hear?
a spoiler?
Or if this was read aloud and I was thinking very literally, I might say -
I"I hear a paradox about hearing and knowing... :PP"
Or if I was being particularly awkward, I'd say -
"What was that?! Speak up!"