When no sound comes out of wired headphones connected to "USB to headphone adapter", how do you get the sound to work?
The only workaround I have found to work reliably is:


restart the phone right before you need to use the wired headphones with the "USB to headphone adapter"
after the restart, the sound always works on my wired headphones connected to the "USB to headphone adapter"


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Side note:


The fact that I have to restart the phone and this causes the sound to always work... IMO indicates a software issue. 
Some sort of Android service that detects the adapter being plugged in is not working properly and the restart ensures that the service is working properly. 
Most likely if someone with better expert knowledge knew exactly what service needed to be restarted... restarting that service would be a better solution (probably faster to restart one service instead of the whole phone... and doesn't interrupt what you are doing on the phone like a restart).


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