1. Make sure all slides are set to start/play automatically (including sound). To do this:
select the slide.
Now you will see Video Tools.
Select Playback
Select Automatically from dropdown menu
2. Play the PowerPoint just to make sure all the animations work and each slide starts automatically.
3. Select File/Export
4. Select Create Video
5. Select Use Recorded Times and Narrations
6. Select Create Video
7. Save in the desired location and wait for PowerPoint to convert to MP4 video