The mp3 music needs to be on a separate track from the audio track that's associated with the video files. THAT track stays UNmuted.
If you'd like someone to check out the issue, here's how...
1. Save your project, then with the project on the timeline, click the FILE menu. Then SAVE PORTABLE PROJECT AS. Save to any convenient folder.
2. Open the folder that contains the saved files and ZIP them all.
3. Upload the zip file to DropBox, Google Drive, MS OneDrive or the like. It's free.
4. Make the uploaded file public (share it) and copy the shared link.
5. Paste the link into this forum.