First of all, you have to know how to open the top shell. The solution is shown here: How to remove top shell of New 3DS XL 2015?.
Now if both left and right speaker works(by any chance because of the first reason) , the ribbon cable should be completely fine, and you probably just didn't align the exterior slider and the interior one correctly. Now in this image
, inside the red circle is the small circuit board that connects to the speaker. You can see there's a little black thing sticking out. That is the "interior" slider, and in the yellow circle which is the "exterior" slider(not the perfect angle but meh you can see it), you can see there's a place for that little black thing to stay. This is how the interior slider move when you move the exterior one. To align them, you simply just have to move the exterior slider to very bottom and do the same to the little black piece, then place that small board back and make sure the little black piece is aligned perfectly. The same is to be done at the other side as well.
However if you tried that method and they are still not working, consider watching a video of replacing the top screen as the ribbon cables are going through the hinge the same way.
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