Why does autism sometimes impede the ability to produce speech? My seventeen year old brother is severely autistic and unable to communicate. He can produce sounds, and imitate words, but he doesn't respond to words in a way that suggests he understands them. His imitations are always imperfect, and this is what interests me most. For example, he can sort of say the first twenty numbers ('ah', 'tih', 'ee', 'hoh', that sort of thing.) He sort of gets rhythm, but not intonation or pitch.

Of course, every child is different, but are there reasons why an autistic teen who can clearly hear words has never in 17 years learned to accurately reproduce them? Dad once speculated that it was auditory, that he can't hear the sounds properly, but I suspect that it's physiological. I'd like to hear thoughts from people who know what they're talking about.
Difficulties with language is not actually a symptom of autism. Autism Spectrum Disorder involves difficulties in social communication and interaction, as well as restricted and repetitive interests or behaviours (DSM-V, 2013). The term "social communication" is referring to difficulties in the social aspects of language and other communication, such as matching verbal and non-verbal behaviour, rather than difficulties in language.

The language difficulties as described in the question sound like they could be related to the age-equivalence of the individual, although of course physiological hearing difficulties can't be ruled out until it is assessed. All children progress through a sequence of language development (Tager-Flusberg et al., 2010):


Pre-Verbal


Babbling, gestures
Typically developing children aged 6-12 months

First Words


Non-imitated, spontaneous single words
Some of the speech is intelligible, and uses consonant sounds typical in babble
Typically developing children aged 12-18 months

Word Combinations


2 and 3 word combinations, including nouns, adjectives and verbs
Typically developing children aged 18-30 months

Sentences


Children create sentences, now using plurals and prepositions
Typically developing children aged 30-28 months

Complex Language


Language for a range of topics, using complex grammar, in different discourse contexts
Pre-school aged typially developing children



Tager-Flusberg et al. (2010) have a great table (Table 1) in their article outlining their proposed benchmarks for each of these phases. As the child progresses, their phonology (speech sounds) improves as well as their vocabulary, grammar, etc. Thus, your brother may be imitating the sounds he hears as best he can within his level of language development.