No audio for mp4 files in Quicktime and ITunes but works for VLC 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.
You can see the link below
QuickTime compatible and non-compatible formats
Codecs supported by MP4 container
TL;DR according to my experience,probably AC3 DTS opus.
Although they are popular formats, quicktime does not support.