Without all the tech details on why my movie audio did not working on your Mac, the simple solution is use VLC as a media player. It is not an incidental program but has a huge following and is probably the best in the free and opensource community.
https://www.videolan.org/vlc/

Tech details on mp4 audio not working on a Mac is a common tread on Google or any other search engine.
Search "no audio for mp4 on apple".