Hi margie251,
Thanks for using Apple Support Communities! I see you're having an issue with recording your voice with the PowerPoint app, and I'd like to do what I can to help out.
You'll want to check that the internal microphone is allowed access for that app:
Control access to your microphone on Mac
You can also test recording your voice in an Apple app, such as Voice Memos: Use the Voice Memos app
Since PowerPoint is a Microsoft app, contact them for any additional help. I found this link, and it may help to get you started:
PowerPoint 2016 for Mac Help
Take care!