I’m using “speaker” in a loose sense here. I don’t just mean people who stand on a platform and talk to multiple people at once. I think most people know that telemarketers are advised to keep a mirror on their desks and smile as they talk to prospects on the phone, even though the prospect cannot see them. When people can see you, your appearance affects how your message is received. But it also affects how you feel about yourself as you deliver the message, which may have as great an effect on ultimate reception.
This concept is explored in some depth in Dressing Up the Brain: Wearing a Suit Makes People Think Differently found in The Atlantic. Check it out.