"They do not hear the dog."
Translation:Nie słyszą psa.
Because even 3rd person pronouns aren't needed, if you have context - and in real life, of course you have it.
If the subject was mentioned previously (Adam and Marek are walking down the street), the next English sentence will use "They" (They do not hear the dog), but Polish rather would not, as the subject is already known. That's why "Nie słyszą psa" is perfectly fine.