I don’t have one. I always thought an inner voice was just a narrative device to deliver exposition in TV shows and movies. It seems weird that most people are constantly talking to themselves. I only have an inner voice when i’m reading or imagining conversations with people. outside of that i just don’t need it.
It does seem like it would be inefficient to put thoughts into words just to think them, but what i’ve gathered from talking about this subject over the years is that for them, the words are the thoughts. Instead of translating abstractions into language they translate language into abstractions. People with very active inner speech might think “I should go to the park today” where I would just think I should go to the park today.