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NAILED IT!
tear apart the case and find the little screw between volume up and volume down (as I illustrated in my picture with an arrow).
I loose that screw to half tightened then all the buttons work again.
finally you put the case back and the problem should be fixed.
I followed this youtube video to tear the case of my Flip 3
title: ***JBL Flip 3 Teardown and USB Repair***
https://www.youtube.com/watch?v=tR7RvDwhwCM