I created a PowerPoint and added an audio narration to the slide show. On playback, there was a noticeable hissing sound.
I search Google to find a solution. Many users encountered the same problem. They offered quite complicated solutions involving third party software. It was too troublesome for me.
Someone suggested to use a good microphone. I re-recorded the narration with a Logitech mic (the best that I got). I passed the completed work to my colleague to convert into a movie and upload to Youtube. It came out perfect. There was no hissing sound.
A week later, I recorded the audio narration to another PowerPoint with the same Logitech mic. The hissing sound was bad. I re-recorded it a few times. It was still bad. On my last attempt, I switched off the air conditioner in my room. There was still a hissing sound, I converted the PowerPoint into a movie file. The movie file has the hissing sound.
I uploaded the movie into Youtube. Surprise! The hissing sound disappeared.
My conclusion - Youtube (owned by Google) had the technology to remove the hissing sound. I am puzzled that Microsoft did not use that technology to remove the hissing sound in the audio recording of their PowerPoint.
Now I know why Google is worth many times the market value of Microsoft. Clearly Microsoft has not kept in touch with the needs of their customers.