I'm not even sure what to do anymore. My last note 2 did almost the same thing - although sometimes the problem would manifest itself by just not making any sound at all (via either speaker). Usually it'll just randomly happen, and I've had this phone almost a month working perfectly - no new apps have been installed in the past 2 or 3 weeks.
It seems the only thing that causes it (and the last one) to act up is if I have the ringer on normal volume for a few days, then magically one day I'll go to unlock it and be surprised by crazy sound. I thought it might be an app (when I had the last phone), so I did a factory reset on it and the problem persisted then too, that's why I had it replaced with this one.
Do you suppose the battery or the back cover could cause it? I've had the same one on both phones as this was a refurbished replacement.