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You can still develop speech if you're missing the auditory part of it (deaf people learn sign language the same way other people learn to process speech, just with visual info rather than auditory). So it's not unreasonable to me that you could be fine with just auditory too.
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That being said, I don't think anyone is wearing a mask 24/7, so parents are still looking at their babies without masks at home, right? Plus TV shows and movies are still going to be a source of seeing people making speech sounds.