How do you change your speaking tone of voice?
To naturally lower your voice, ironically, look up.
To sound more serious, slow down your speech, pronounce the start of each word more fully, and whenever you can, change questions into statements/ commands
E.g. %26quot;is it ok if we go?%26quot;
becomes
%26quot;Lets go then.%26quot;
obviously don't do all this so much it sounds fake or stupid- just gradually start using these techniques
