My ds has had hearing tests from about 16 months due to constant ear infections/glue ear.
He didn't cooperate at all until he was over 3yo, and, from what they said, this is very normal. The first time he (sort of) cooperated as far as they could see, I could see that he wasn't actually doing the hearing part.
He had to put a passenger on the bus when he heard the sound, and the first couple of times, he stopped, looked at the tester, and she said "if you heard it, put it in the bus", after that he just waited for her to say that then put it in the bus. 
The next time, (different person) I told them this had happened and he managed to do enough of the hearing test to show them roughly his hearing-he was just younger than 4yo then. In the last year (age 4.6-5.6) they now tell him he's a pilot and has to press the button to fly the plane when he hears the noise and he absolutely loves it, hence we get a great test. But, from what is said, it's not totally uncommon for even this age to not want to cooperate on the test.
Lets face it, the soundproofed room feels funny, plus the headphones are scary to a child who's not used to them.
You may be able to help there if you can get some big earphones and make wearing them a game, perhaps?