Hi,
Thank you for writing to Microsoft Community Forums.
I understand that you want to add background music for parts of the video using Microsoft Video Editor. I’ll
certainly assist you with the details.
Upon researching I found that currently, we do not have the feature to add background music to parts of the
video. However, you can add a custom audio and then drag the audio to a particular section of the video where you want to add it.
To do this, click on Custom audio on the top right section in Microsoft Video Editor, then click on the
+ Add audio file and select the audio. Once you add the audio, you should see something like in the screenshot below:

You can drag the Audio to the section, as per your requirement.
Let us know if you need any further assistance, we will be glad to help you.
Regards,
Nikhar Khare
Microsoft Community - Moderator