Voice conveys personality and point of view, while tone conveys emotion or attitude.
The former is consistent, the latter varies.
For example, let’s say you are someone who likes pizza. You’ve eaten it many times, you have a favorite topping, a favorite pizza joint, and so on. You are a pizza proponent. A voice for pizza, if you will.
However, your emotions toward pizza can depend on the mood you’re in. If you haven’t had it in a while and suddenly decide to order it for lunch, you may happily exclaim, “I want pizza!” But if you’re on a diet, you may say regretfully, “I want pizza…”
If you want a less contrived example, consider this: we speak to our friends differently than we do our parents or our colleagues. Our core views remain the same, but our word choices and tone of voice may differ. Asking your boss for a raise? Turn on that professionalism. Telling your best friend her new boyfriend kinda sucks? Get out the tough love.
The gist is that you stick to a point of view, but change how you present it based on what will resonate positively with your audience.