I was able to solve the problem myself. In my case, it was a setting. When I opened the video, the audio volume was turned all the way down by default. I still don't know why it happened, but turning up the volume fixed my problem.
I am also having this issue. I recorded two portions of a meeting and I had 5 files, two videos, two audio, and a .conf. The audio and video files have no sound. I tried adjusting the volume but no luck. Has zoom released any updates on this issue?
I had the same problem recently and found that the problem is in my computer sound settings. Try to check if your output device is the same as the one you set in Zoom. In my case my output device was different than the one I set in Zoom and once I corrected it, I heard the audio in the mp4 recording. Hope this helps!