I'd say there is definitely some interpretation, but I'd say it would depend on the injury, the source of the injury, and the nature of the magic used to restore it. Greater Restoration can fix most things, up to certain powerful curses and such.
I'd say it could restore his ability to speak, though it may be fun to explore the situation that leads to the his regaining the ability to use his vocal chords and how that works out afterwards- the experience of speaking again after so many years.
You could request a short CDQ from a GM, or just maybe schedule an non-GMed event for other PCs to be part of the journey to recover his ability to speak (so you don't have to wait for a GM). Then you'd have a story to tell about it when he got his voice back. Alec's New Groove.
