i have a problem with the mp4 video encoding in Mac

[Julio Ernesto5C9A]

when i export long video like 20 mins or more the audio in the middle of the video get muted from the middle to the end and it only happens when i export to mp4 if i export it in mov i dont get this problem, plz someone help me (srry for my english i know is bad lol) 

[Jeff Bugbee]

What format video/audio are you using? Usually issues like this are due to compressed formats.