I haven’t had this problem until today.
I import an MP4 file into Audacity.
I deleted the opening and the dead space at the end.
Everything plays fine and I export it as an MP3 file.
Quicktime plays the new file.
However when I import it into Apple Music, I see the file and if I Get Info, it has the correct length. However it will not play when I press the Play button. And maybe this is significant, but it won’t open in VLC either.