I have some movies in mp4 format. They work perfectly on VLC (audio and video) but not with QuickTime and iTunes. I can only watch the video but without audio! I have a MacBook Air 2017.
MP4 is a container format,the audio codec format could be aac mp3 ac3,etc.
iTunes and QuickTime can’t support all those codecs.You can use mediainfo to check the exact audio codec format.
TL;DR according to my experience,probably AC3 DTS opus.
Although they are popular formats, quicktime does not support.