To export, on Mac, choose File > Export
Select File Format to MP4 and change quality to "Internet Quality"
this will keep the file small, so you do not have to compress it later! :-)
How to record your presentation including your voice in PowerPoint (Apple)
(This is the Apple version, for PC go here).
This guide will show you how to record your presentation including your own voice and then export the whole thing as a video.
Great, now continue to step 3 !