This is the 11140th most frequent Turkish word.
duymadığını
"Duymadığını" means "that you/he/she/it did not hear."
In this sentence, 'duymadığını' is utilized to express the third person singular did not hear 'duymamak' in an indicative clause of 'that they did not hear', showing negation.
Söylediğim şeyi duymadığını fark ettim.
I realized that you did not hear what I said.
Here, 'duymadığını' indicates the action of not hearing by the third person in an indirect manner of questioning in a subordinate clause.
Onun duymadığını biliyor musun?
Do you know that he did not hear?
In this usage, 'duymadığını' serves to share reported speech involving negation about the third person's ability to hear in indirect context.