We can break down the difference between a monotone voice and a dynamic voice into two basic parts (although there are many more divisions we can use, we’ll stick to the basic/obvious ones).
These two parts are the most commonly known features of sound:
Amplitude and Frequency.
Amplitude being volume. A dynamic speaker would vary their volume throughout their presentation to maintain interest and emphasize important points. Whereas immediately you can tell a monotone speaker most likely maintains the exact same volume throughout their ear-bleeding sleep-inducing presentation.
Frequency being pitch. Similarly to variance in amplitude, a dynamic speaker would vary their pitch delivery of the presentation for the same reason; to maintain an interest and a dynamic “flow” to their presentation. One person that I always strive to emulate when making presentations is Steve Job’s keynote speeches. You can tell he spent lots of time rehearsing and rehearsing. Not only his actual script but just as important was the synchronization between his scripts and the keynote presentation on the screens.
Next time you hear a monotone speaker, try to see how little they vary pitch and volume.
Don’t forget: pauses are just as important! Adding pauses before/after important or difficult concepts allows the listener to absorb and process your content!