Good day
Thank you for using Microsoft Community!
I understand from your description during a trim, there is no sound in your Video Editor.
We tested your situation by recording a .MP4 video by camera and trim it in Video Editor.
And we found that, during trimming, there is still sound in the video.
You may please right click sound icon in the right corner of the button and Open Volume mixer to check if there is sound when trimming the video.
And could you please tell me is the specific .MP4 file lost sound or all media files lost sound?
We look forward to hearing from you.
Best Regards,
Silvia | Microsoft Community Support Specialist