I have an audio track set to start at the beginning of my animation, and when testing the video as a SWF and previewing it it starts in the beginning as it should, but after converting it into an MP4 it wouldn't start until ~43 seconds into the video.
The best thing to have an MP3. Also, make sure your audio settings is on stream not event. Make sure the audio is exactly where you wanted to start on your timeline. finally, make sure you have the latest software adobe animate 21.0.3.