In summary, speech is not central to language.
Here are some highlights of myths and facts on speech and language.
Assumption: hearing people commonly believe that speech is superior. They believe that speech is the avenue to language acquisition. They believe that speech and language are directly linked.
Fact: Neruscience and linguistics studies defy those beliefs, showing hard science evidence that language is amodal, which means that visual-spatial modality is on a par with vocal-aural modality, and language is brain-based.
Assumption: Speech is paramount to language acquisition.
Fact: Language acquisition studies show that both visual-spatial language (e.g. ASL or any signed langauges) and vocal-aural language (e.g. English or any spoken languages) are on the same timeline of language acquisition milestones from birth from babbling to two-word stage.
Assumption: The auditory tissue in the brain was thought to be for auditory processing. "Exposing a child with a cochlear implant to sign language will hurt 'auditory' tissue development."
Fact: Hearing/eyeing speakers process signed language and spoken language in the "exact same tissue". It has nothing to do with sound; rather, it has to do with the patterns of language.
Assumption: Deaf children (with cochlear implants) must have an intensive speech therapy and sign language must be avoided at all costs, because early exposure to sign language will impair the acquisition of English and/or speech.
Fact: This speech-only approach more likely may impair language development. Speech is not a language. It's a modality. The crucial thing is to develop a language regardless of modality. Bilingualism (ASL and English) provides deaf children language acquisition and the benefits of being bilingual and bimodal (e.g. visual processing advantages that hearing children lose). Bilingual-bimodal children have stronger language skills, even in reading.
"Researchers have found that if a child has none of that training, if a young child does not have that phonological training, if they only have ASL phonological exposure, they end up better readers and writers. This provides evidence of the fact that ASL does not harm one's ability to develop literacy or skills in English." (Petitto) I can attest to the fact that many bilingual-bimodal children (whether hearing or deaf) of bilingual Deaf ASL-speaking parents have advanced reading skills.