In Gathering Storm there is a reference to a Deafened condition, but I can't find this condition.

Where is it?

Bit of a guess, because i don't have stuff readily available, but could it be referring to the ThunderStruck condition that comes with the set?

Good question, just come across this myself. I've extrapolated from the 'Blinded' condition by adding an extra challenge dice for hearing-related tests. Will also apply this to initiative if it seems appropriate to the situation.

Pumpkin may be right re: Thunderstruck, but if that's the case it seems a bit OTT for a loud crack of thunder to cost you an expertise dice on every skill check and +1 damage on every successful attack against you, IMHO ... that seems more in line for one of them taking a direct hit from a lightning bolt!