d9780 Have you made sure to check your USB devices settings? Some people may choose to block all on accident.
I had some issues with plug-in AKG earphones on my 6a. Sometimes they wouldn't play through it at all, I just end up plugging in and out and eventually it plays. Sometimes it would say it is 'Charging device by USB' as an indicator it wasn't plugged in properly.
Other than that, your earphones could not be playing nice, try another device, and if they work perfectly fine elsewhere, try and look at other earphones to see if it might be a USB port issue?