A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Some versions of PPT allow you to mark a shape as Decorative when editing the Alt text info.
That would be one solution.
Is the goal to make the presentation accessible, or is it to pass PowerPoint's accessibility checker?
The two aren't entirely the same.
If the goal is to make it accessible, you can simply ignore the warnings about the video if all that matters is the sound.
Or use a video editing program to split the audio and video tracks, so you can insert just the audio. That should also make the accessibility checker stop barking at it.
If you don't have a video editing program that can do this, Audacity, a free sound editor, can be set up to record system sound; do that, then play the video and you can record just the sound and save to an audio file to insert into PPT.