User can do many things with their voice

Pros:
-user can hypnotise others
-user can charm others
-user can change voice easily
-user can use magic/energy based attacks/defence
-user can make shock waves
-user can control others
-user can create illusions

Cons:
-user will lose voice with constant use
-user must user spell correctly or it will go wrong- horribly
-user will weaken with a lot of powerful spells

Weakness: overuse
User loses voices completely and won't be able to walk