Hmm, that's interesting, I would never have guessed that the manner of speech is something that might need improvement. I guess the points you mentioned apply to any job (and probably gender), though.
To add to this, I know that when I'm nervous I smile a lot and laugh to try to ease my own tension, so I can understand how an interviewer would take that as being naïve or not serious about the role, when it's really just a case of anxiety mixed with imposter syndrome.