I have no idea when this started as I wasn't using the headphones on the laptop for long time - so it might be since the latest minor upgrade.
After listening to music for a while, the sound starts shifting the balance to the right as if someone was slowly moving the balance bar. Rarey I can hear it moving the balance left and right for a couple of seconds.
Please note this is not the same issue recognised by Apple where opening the sound preferences you suddenly see that the balance is actually shifted to the right (or left). My sliders are always perfectly centered.
Sometimes when it happens and I go to the "Audio Midi Setup" and I move the left slider, the balance restores to the center, but other times it just mute completely the left channel. When this happens, also the test sound in the "Configure speakers..." section stops working, for both channels even if the right one is still playing music.
Sometimes plugging out the headphones and plugging them in again restores the balance, other times it doesn't.
This also happens if I use an external USB amp/dac so it's not the headphone itself nor the jack.
I tried rebooting without re-opening the apps, 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?) and NVRAM, several times. Killing coreaudiod too doesn't work.
I used Onyx to clear the system caches (including Kernel and such) and rebooted, still nothing.
I wouldn't like to upgrade to Sierra yet (assuming it would possibly fix it) as it seems there are too many bugs and I need the laptop to work.
Any idea, please?