Question: No sound playing MP4 on QuickTime
Certain MP4 files do not play with sound through QuickTime. I downloaded Elmedia Player and the sound came back. It's too bad that I have to rely on external software. The same MP4 files play fine on the Windows Media Player on my PC.
I just upgraded my macOS to Mojave version 10.14, but this problem existed previously as well, with High Sierra.
MacBook Pro with Retina display