Good vocal variety is the best tool in your public speaking toolbox. It helps keep the audience engaged, provide them with your meaning and emphasis, and activate pathos part of the rhetorical triangle. You could learn to use it effectively to enhance your presentation, just need some practice. However, preparing both content of speech and practicing vocal variety could be hard exercise. One possible solution is using existing text to read them out loud, similar to the Toastmasters legacy advanced manual “Interpretive Reading”.
One excellent source of “fake-it-until-you-make-it” texts is famous letters. Project Letters of Note compiled a collection of the world’s most entertaining, inspiring and unusual letters. It is available in a book form of book and in an online-collection. It was extended by a Letters Live project, a series of live events where remarkable letters are read by a diverse array of outstanding performers. You could find many videos on project’s Youtube channel and use them for practice.
Select your favourite letter and read it out loud to improve your vocal variety! (NB: My favourite is Benedict Cumberbatch reads Sol LeWitt’s “DO!” letter to Eva Hesse)