A variety of characteristics are used to diagnose autism in children. Often, it's poor social and communication skills which others observe in children that compel parents to get a child tested. There are currently no unique biological indicators of autism.
One particularly rich indicator of social differences in autism is the voice. Children with autism often sound different from other people. Some may speak in a flat, monotone voice; others may use unusual modulation or stress different words or parts of words in their speech; and some may speak at an increased volume.
Children with autism also may understand an emotion’s meaning differently when it’s conveyed through tone of voice. But that’s not because they always have trouble producing emotion in speech; the difficulty is that people with autism have problems understanding how tone is used to modify the meaning of words.
So, for example a child with autism may have difficulty parsing the difference between the sentences: “I can’t believe this” and “I can’t believe this.”
Their difficulties in understanding tone and their differences in vocal and facial expression may affect how others form impressions of people with autism. In studies, so-called neurotypical people have reported being less inclined to interact with people with autism after hearing them speak.
As it happens, people with autism are just as likely to judge vocal expressions of people with autism as less natural. That means that both are sensitive to the fact that tone of voice can bring on judgement from other people. But more familiarity with the people on the spectrum helps to minimize negative bias and helps make richer social interactions possible.