3D Slider and Volume slider issues

I bought a used 3DS XL a couple days ago.

When I first got it the volume seemed a bit low - at least lower than my small OG 3DS, but it wasn't until I got home that I noticed that the volume actually becomes LOWER when the 3D is on - and it gradually goes up when the 3D is turned off.

Not to mention the volume slider is kind of unresponsive - if I slide it all the way down it takes a while for the volume to become lower (and even then, the music can still be heard, but barely). Same thing happens when I slide it all the way up, it takes a bit for the volume to gradually become higher, and it's not that high compared to my other 3DS.

The only way to make it change the volume faster is by pressing Left on the D-Pad (somehow).

I've opened the console up and there doesn't seem to be anything out of place or broken, but the problem persists.

What I'm asking is, which part do you think is the culprit? The volume slider? The 3D slider? Both?