After export, you may see one of these notes:
"This video has no sound. To include it, export from Chrome or Edge on Windows or Mac, or from Safari."
"The sound could not be read, so this video is silent. Try again, or export from Chrome or Edge on Windows or Mac, or from Safari."
"Instagram and TikTok can reject the sound in this file. For a file every network accepts, export from Chrome or Edge on Windows or Mac, or from Safari."
Some browsers and systems cannot write sound in the MP4 format networks expect.
What to do
Open the same video in Chrome or Edge on Windows or Mac, or in Safari, and tap Export MP4 again. Your video is saved, so nothing needs rebuilding.
Also check the Audio panel: Voice-over and Clip sound should be above 0%.