Audio shifting balance to right headphone (MacBook Pro Retina, 13-inch, Mid 2014 - El Capitan)
In your question you state:  I've reset the SMC (actually I have no way to know if it worked as the green light in the power plug did not flash?)

So, just to be sure you've used the correct procedure for your particular MacBook Pro (i.e. one with a built-in battery you can't remove), the steps for resetting the SMC are:


Shut down your computer
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button.


Hope this helps.