If you uploaded a video to Facebook and the audio got muted, that's because your video may have violated a new Facebook policy for videos that contain music.
Facebook no longer supports videos with music if the video doesn't change. For example, if your video is only the cover art that never changes, then Facebook will mute the audio in your video.
OK, but what should I do now?
Option 1: Update your video so the visual changes and is not just an image.
Option 2: Post a *story* instead of a video, then attach your music to the story. (When you post a Facebook or Instagram story, you can use a single image and then attach your music using the new music features)