I'm comming at this from completely the opposite direction. I have a friend who is deaf, as is her husband, her parents and two of her four children.
In her family deafness is normal, it's the two hearing ones that have been a new experience.
I'd second the NDCS - they have lots of information and loads of things for children too - camps in the summer with activities, or drama.
Do you need to know the cause? If it is genetic will that stop you having more children? If no cause is found will that stop you having more children.
For what I know about hearing parents who discover their little one is deaf / HI, there is a period or mourning for the little girl you had. But yuo still have your little girl, you just found out something different about her.
Jodi Picoult has a scenario - given to her by the mother of a child with Aspergers
Wanting a child is like wanting to go to Rome. You read anout Rome, you dream about visiting Rome, you learn some Italian, you get recomendations from friends. You fianlly get on that plane to Rome and you are so excited.
But when your plane lands you are in Paris, and you can't get to Rome. Everything is wrong. Nothing is how you expected it to be. You don't know a word of the language, you don't know if you will lie the food, this is not your dream.
But than, after a few days you realise Paris is beautiful. There are flowers. The food is different, but good and eventually you realise you don't actually want to go to Rome at all.
I'm not in your posiition, I do not know how I would react if I was. I hope you discover Paris is beautiful.