I've been told that ABR is quite accurate on small babies, but NOT for toddlers. The last time I asked for it I've been told that after he's 1yo it wouldn't be accurate and may lead to false positive/negative responses. After 18 mo they told me the only accurate exam is electrocochleography (oh gosh, is this even a word?? Hope it make sense to someone, it's the exam with a needle in your eardrum), but we never went for it since it's invasive.
I guess the best you can do is wait for the kid to cooperate to the testing.
And well, talking about autism just based on one test is ridiculous. There should be a little more to notice, you know (my son hase some autistic traits, but I'm not worrying since I did have same as a child, at the moment he seems to be able to overcome this with just minimal help). Not talikng at 1 y and 1/2 is not so uncommon. Does he make any sound at all? Babbling? How does he behave in everyday life, like when you call him, talk to him etc? Soundbooth test... Really NOT reliable at that age. My son is 3 and still tests are not reliable at all, they told me we must wait at least until 5 or 6.
I'd have him tested for autism in some place where they KNOW what to look at. Some doctor who works in that fields. But if there's a relational problem and not only a hearing one, just look at your kid and you'll notice. I know I did... Spinning, flapping hands, lateral look, particular night awaking patterns, putting things in rows, great sensitivity to neon lights, are all more reliable as synthomps than lack of speech at that age. And still, thay're not enough for a diagnosys.
