Begin by going to the slide in your presentation that you want to modify, and select the Animations tab. Click the arrow on the Transition Sound drop-down menu to expand the selection area as shown below. (Click any image in this article for a larger view.)

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Holding the cursor over any of the sounds in the list will let you preview the selection.

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When you’ve found the sound that you want to use, click on it, and it will be added to your slide transition. You can then click the Preview button to see how the slide along with the animation and sound will be presented in your slide show.
If you want the sound to continue to repeat itself for as long as the slide remains the focus of your presentation, select the Loop Until Next Sound option from the Transition Sound menu.

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If you do choose this option, make sure to view your entire slide show before your presentation. It doesn’t take much for an audio special effect to become extremely annoying to your audience. The last thing you want is for all your viewers to leave your presentation with nothing but the memory of an aggravating, repeating noise stuck in their heads.
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In addition, if you're a regular user of PowerPoint, you may want to take a look at these tips that will help make presentation creation more efficient. Here, you can find sources that will detail how to create macros, how to use the same slides for multiple presentations, and more.