Exporting movie from Keynote SOMETIMES doesn't include audio

I'm running Keynote 13.2 on a MacBook running OS 14.7. I record lectures in a class, then export them to a movie for students to view online. Out of about 30 lectures this semester, 4 of them did not include the audio in the exported movie. I have no idea why. The audio is there, when I play the recording within Keynote, but that audio is not included in the mp4 output. Help!