Changing the tone of your voice by adding emphasis to words or phrases will help the audience grasp the importance of what you're saying. It will also give them a mental cue that something has changed and to pay closer attention to find out what it is.
You don't have to yell or scream at the audience, simply just say something a little louder if its "BIG" or a little softer if "small." Changing tone of voice will also add to the humor of a presentation loosening the audience up and making them feel more relaxed with you as a speaker and with the information you're showing them.
Only emphasize words or change your tone of voice when it's necessary. Don't say something louder just to be louder. Make sure you're actually emphasizing something that is more important or warrants being said differently than everything else. Your audience will become confused if you start to randomly become loud and then soft for no apparent reason.
Here are the best places to change the pitch of your voice:
#1: When you have something important to say
#2: During a story
#3: At the conclusion of the presentation
#4: If the audience's attention drifts away
#5: After a period of heavy informational intake