William pointed me to a BBC report on gene therapy for the blind. I am sure it's still decades away, and might not even be possible theoretically. The intervention is probably very local, but the damage to the speech system and possibly adaptation is distributed.
Think of bad genes as mistakes in a construction plan for a house. Once the house is built, correcting the mistakes are not helping. Of course, the brain is a bit different, because genes instruct the cells to build proteins for a life time. However, the main fibre structure are set at early age and never change again.