In the absence of voice intonation or modulation, people will give your message a negative connotation.
Therefore you must have some fluctuation and variation in your voice.
The best way to practice varying your voice is to read children’s books out loud. Make sure you adjust your voice for each character and that you clearly articulate each word. Practice with a tape recorder so you can listen to yourself from the audience’s perspective and improve your presentation skills.
You can also think about a subject you are passionate about and then talk about that into a tape recorder. Listen to your voice fluctuation and see how it changes as you talk about something you have a lot of passion for. Then take any piece of reading and try to read it with that same passion.