Hello! I'm Jen, and I'd be happy to help you out with this issue.
I’m sorry to hear you’re experiencing this issue. Please make sure you have enabled the option to allow Microphone access to Powerpoint. To check, go to Settings > Privacy & security > Under App permissions, select “Microphone” > Switch on "Microphone access". Under “Let apps access your Microphone” section, toggle on the switch for Powerpoint.
If the problem persists, try to run the “Recording Audio” troubleshooter and see if it fixes the issue. Open Settings > System > Troubleshoot > Other troubleshooters > Run the troubleshooter for “Recording Audio”. Relaunch Powerpoint to check the result. Let me know how it goes. Thank you.
I hope this information helps. If you have any questions, please let me know and I'll be glad to assist you further.
Best Regards!
Jen :)