Most people understand that non-verbal signals (facial expression, body language) and voice tone have a strong impact on communication and how the communicator is perceived. In a teleconference, these non-verbal signals are missing so voice and tone become even more important. Listeners naturally use their imaginations and fill in the gaps. A presenter in such a virtual environment must compensate for missing signals by adapting his or her style to make a positive impression.
Even though no-one can see you, sit in a relaxed way but with a good upright posture. Smile and use facial expressions, gestures as this has a positive impact on the voice and animates tone. It is essential to speak slowly and clearly, projecting your voice forward and using vocal variety. Asking rhetorical questions is one way to do this as they impact voice tone AND keep the audience involved. Finally, keep slides simple and free of animation and transition effects and link your words closely to the visual support.
In this way, you should be able to make a good impression!