Your voice isn't flat because of how you sound, it's flat because you're not using it. Five levers control it: pitch, pace, pause, volume, and tone. Pick a line from your next talk and practice each one until it feels natural.
Chapters
If you've ever been told you sound a bit flat, don't worry, it's not your voice, it's how you're using it. So great speakers don't just say things, they use their voice like a musical instrument, and they use 5 levers to do that. First of all, pitch. High equals excitement, low equals authority. Then you've got pace. Speeding up for energy, slowing down to land a point. Then you've got the pause. Silence isn't awkward, it's power. Then we've got volume. Project to command. Lower your voice to draw people in. And then finally, tone. It's not what you say, it's how you say it. There you go. Grab one line from your next talk, have a practice saying that same line using those 5 different levers, and once you get more comfortable, then your speaking will improve. Enjoy. Woohoo!