Would good old Siri work for you? It seems to verbalize + - * / answers. it adds "It looks like the answer ..." or "let me check on that" when you add a decimal, which can be annoying if you are trying to do more than one calculation in a row. Percentage answers were only written not spoken. Plus you would be constantly tapping to talk which ain't hands free. Good luck in your quest.
Siri just got impatient with my calculations and stopped adding up, giving me the final equation far too soon. Not reliable I guess.
Maybe try the google version? You can download for iOS and it has voice function, might be better for what you are trying to do. The app is simply called "google"
“If you are neutral in situations of injustice, you have chosen the side of the oppressor. If an elephant has its foot on the tail of a mouse and you say that you are neutral, the mouse will not appreciate your neutrality.” Bishop Desmond Tutu