I have a 15 minute PowerPoint presentation, together with an MP3 narration playing across the slides from start to finish. It goes well; all that remains to do is convert it to mp4, and it should be just as good on video.

No such luck. PowerPoint 2016 creates 'echos' in the audio file. By 'echo' I mean, some of the words are repeated twice - like an echo. The effect is a cumulative lengthening of audio, gradually getting out of sync with with the transition timings.

Is there a workaround for this, or does another version of PP do a better job?