Even when the two dementors are still a good distance a way from him, Dudley quickly becomes a sobbing wreck in the first chapter of Order of the Phoenix. Why is this? As I recall from Prisoner of Azkaban, this is not a normal reaction to dementors. For example, when there was a single dementor on the train in Prisoner of Azkaban - which I imagine to be much closer proximity than what we had at the start of this encounter in Order of the Phoenix - Harry, who is known for being very sensitive to dementors, is the only one who has a reaction on the same scale as Dudley's.
Preemptively, I'd like to state that I'm not asking what he heard. This is only a question of comparative dementor effectiveness.