How to make people take you seriously: 5 powerful psychology-backed tips

A 2017 research article from the National Institutes of Health’s PMC repository (Tjaden, 2014) discusses how clear speech (which is characterized by slower rate and hyperarticulation) improves intelligibility and the effectiveness of communication, which can indirectly influence perceptions of confidence and competence.
Another study from Columbia University found that a measured tone signals authority, while a rushed or shaky voice indicates nervousness.

So, if you want to sound more confident and authoratative then you need to practice speaking slowly, and avoiding filler words. This will improve your clarity and make your speech more impactful.