The 3ds depth potentiometer is connected to the speaker flex ribbon connector which connects here http://www.consolezombie.com/Images/3DS/...
the replacement is very difficult. the speaker flex is like 3 dollars but you have to solder your own speakers. if that one flex is broken i bet the other might give out.
my suggestion reseat the ribbon connector for the speaker wire. And if that works then dont close your 3ds anymore.
dont even go into the top lcd parts unless you watched several videos and already purchased replacement parts and have fixed phones before. the cables have to be wound up to fit throught the hinge part difficult and the other hinge has a lock part.
ps careful when lift in sd card port u dont want to life the metal underneathe it. you dont need to do that. but if you do i suggest putting the screws back under the sd card slot to hold the metal part down while you lift off the sd card from the adhesive. only need to do this first time