I have had exactly the same problem with two Idol 4S now. Both model 6071W. (For those interested, the model # is etched into the glass on the lower back of the phone - have to look at it in bright light at an angle.)
Both times, it seemed to happen one of the times (not the very first time) when the phone was 'waking' after it had updated to 15063.251. I called Alcatel support, and the rep didn't seem very surprised, asking 'does it sound like the speakers are blown?' once I started trying to describe the problem.
I want to use Windows 10 Mobile. I like this phone - but if the next RMA fails again, I'm moving on... but to what? There so few Creator's Update compatible new Windows phone devices for the US market...