To increase vocal volume, don't just try to be louder. Instead, pick a specific spot in the room (e.g., in front of the furthest person) and aim your voice there. This mental trick naturally adjusts your posture and breath for better projection.
Your voice has limited stamina. To project consistently during a long presentation, you must train it beforehand. Practice by reading out loud for progressively longer durations (e.g., 5-10 minutes daily) to build the necessary vocal endurance, just as a runner trains for a race.
To improve your breathing for speaking, practice by reading any text out loud. This acts as a proxy for speaking but frees up your mental energy from having to formulate thoughts, allowing you to focus exclusively on the mechanics of breath control.
When using a handheld microphone, maintain consistent volume by lightly holding the mic and anchoring your elbow against your body. This creates a stable frame, keeping the mic a fixed distance from your mouth, while you gesture freely with your other hand.