My hearing is very good, probably better than that of many my age because I didn't ruin it in my teens with headphones and loud music. But I still find that I can understand what people say better if I can see their faces. Now I know why.
A Canadian study indicates that there is a signficant visual component of language understanding even amongst hearing people. The four-month-old babies in the study were able to distinguish when adults in a silent video were speaking English and when they were speaking French.The eight-month-old babies could do the same, but only if they were being raised in a bilingual French/English environment.