PowerPoint Online plays music and other sound effects that you’ve added to your slides using the PowerPoint desktop application, but you can’t add them from within PowerPoint Online yet.
If you want this feature added to PowerPoint Online, please go to our suggestion box and vote for it.
If you have the PowerPoint desktop application, you can add sound effects there. Here’s how:
In PowerPoint Online, click the Edit Presentation tab, and then click Edit in PowerPoint.
In the PowerPoint Online desktop application, use these articles to learn how to add music and sound effects to your presentation:
Note: When you save the presentation, it is stored to the same location you opened it from in PowerPoint Online.