Loudness is often recruited to do a job that confidence would do quietly. Someone who is certain has no need to fill the room, so they leave pauses, speak at a normal volume, and let the point stand, and everyone reads that as authority without quite knowing why. Force reads as needing something. Calm reads as having already decided.

Practise ending your sentences and stopping, rather than trailing into more words. Lower the volume slightly when the subject gets important. If a point needs volume to survive, work on the point instead.