I do some incantations or chanting in my mind and they work just fine. What matters is your intent, wish, preparation, mind set, etc.
I don't think being deaf could be a problem - in magic you work with energy and for her (energy) there is no difference in language.
What I could predict, that chanting aloud could help dealing with energy. But that's probably it.
If you believe that chanting in mind will work - than it will ;)
Intention is key. Spells are not limited to those with deafness. Spells can be said in the mind. As long as you have true intention, knowing what you want and knowing what you want to say, it will work. You need not worry.