I was having an issue this morning with my Moto G4 and thought there could be more victims.
I could only hear the sound from the left speaker of my headphones.
So what was causing it?
I figured out a solution.
At first I wanted to try and fix it physically with the help of this tutorial and as I was building a thing out of cupboard I went to search the internet again.
And then I stumbled about this where someone said, restarting the phone made it work again for him.
This fixed my issue so I came to the conclusion that this is a software issue/bug.
I hope this helps somebody else.
For me it was very annoying because I only have this phone for two weeks. So restart your phone every once in a while or you won't hear anything soon ;)