Under "Developer Options" (https://youtu.be/gn4bRTFicZw) scroll down to Media / Media transcoding settings and disable "enable transcoding" and "assume apps support modern format." This will not work for videos already recorded but will work for future media.
Oh my god I've been having the same issue for weeks. I've tried to speak to Samsung and they just told me to put my phone on safe mode but it hasn't done anything, and I have to send videos on there that are edited for work. I spoke to WhatsApp and they're saying the issue is with Samsung. But they're not being very helpful.
In the editor, click the three dots on the bottom right, then click create movie, then add videos and edit from there
Somehow there is a bug that the sound goes missing after edited videos are sent through WhatsApp, but it doesn't apply if we create new movie.
Samsung, please fix this