Monica,
If you are talking about electronically altering your voice, that is very irritating, but if you are asking if
you should talk monotone or highly animated, then neither of those are right either.
A good speaker knows how and when to emphasis using slight tonal modulation of their voice, or slight volume changes. Think of a good speaker, one you enjoy listening too. listen for how they emphasize and contrast things using voice inflections.
nobody likes to listen to a monotone presentation, but no body like to listen to a person that is inappropriately using voice inflections or volume in their voice either.
Les